[PATCH liburing] examples/zcrx: don't forget to sync dmabuf

[PATCH liburing] examples/zcrx: don't forget to sync dmabuf

[Pavel Begunkov]

dma-bufs are not always coherent and we need to sync it before
accessing. Use proper dma-buf API for that.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
---
 examples/zcrx.c | 40 +++++++++++++++++++++++++++++++++++++++-
 1 file changed, 39 insertions(+), 1 deletion(-)

diff --git a/examples/zcrx.c b/examples/zcrx.c
index b55b07ce..47036a33 100644
--- a/examples/zcrx.c
+++ b/examples/zcrx.c
@@ -39,6 +39,7 @@
 
 #include <linux/memfd.h>
 #include <linux/udmabuf.h>
+#include <linux/dma-buf.h>
 
 #include "liburing.h"
 #include "helpers.h"
@@ -103,12 +104,46 @@ static unsigned long area_token;
 static bool stop;
 static __u32 zcrx_id;
 
-static int dmabuf_fd;
+static int dmabuf_fd = -1;
 static int memfd;
 
 static int listen_fd;
 static int target_cpu = -1;
 
+static void dma_buf_begin_cpu_access(void)
+{
+	struct dma_buf_sync sync = {
+		.flags = DMA_BUF_SYNC_START | DMA_BUF_SYNC_RW,
+	};
+	int ret;
+
+	if (dmabuf_fd < 0)
+		return;
+
+	ret = ioctl(dmabuf_fd, DMA_BUF_IOCTL_SYNC, &sync);
+	if (ret) {
+		fprintf(stderr, "sync failed %i\n", errno);
+		exit(1);
+	}
+}
+
+static void dma_buf_end_cpu_access(void)
+{
+	struct dma_buf_sync sync = {
+		.flags = DMA_BUF_SYNC_END | DMA_BUF_SYNC_RW,
+	};
+	int ret;
+
+	if (dmabuf_fd < 0)
+		return;
+
+	ret = ioctl(dmabuf_fd, DMA_BUF_IOCTL_SYNC, &sync);
+	if (ret) {
+		fprintf(stderr, "sync failed %i\n", errno);
+		exit(1);
+	}
+}
+
 static int get_sock_cpu(int sockfd)
 {
 	int cpu;
@@ -373,6 +408,8 @@ static void verify_data(__u8 *data, size_t size, unsigned long seq)
 	if (!cfg_verify_data)
 		return;
 
+	dma_buf_begin_cpu_access();
+
 	for (i = 0; i < size; i++) {
 		__u8 expected = (__u8)'a' + (seq + i) % 26;
 		__u8 v = data[i];
@@ -382,6 +419,7 @@ static void verify_data(__u8 *data, size_t size, unsigned long seq)
 				(unsigned)i, expected, v, (int)expected - v,
 				seq, seq + i);
 	}
+	dma_buf_end_cpu_access();
 }
 
 static unsigned rq_nr_queued(struct io_uring_zcrx_rq *rq)
-- 
2.54.0

Re: [PATCH liburing] examples/zcrx: don't forget to sync dmabuf

[Jens Axboe]

On 10/16/26 10:25 AM, Pavel Begunkov wrote:
> dma-bufs are not always coherent and we need to sync it before
> accessing. Use proper dma-buf API for that.

Can we get a Fixes tag for this please? Other fixes from today
lacked that as well, please always add fixes tags.

-- 
Jens Axboe

Re: [PATCH liburing] examples/zcrx: don't forget to sync dmabuf

[Pavel Begunkov]

On 10/16/26 17:29, Jens Axboe wrote:
> On 10/16/26 10:25 AM, Pavel Begunkov wrote:
>> dma-bufs are not always coherent and we need to sync it before
>> accessing. Use proper dma-buf API for that.
> 
> Can we get a Fixes tag for this please? Other fixes from today
> lacked that as well, please always add fixes tags.

There was no "fixes" today. I didn't know you care about
liburing fixes tags either, but if you want to put one:

Fixes: 93d3a7a70b4a6 ("examples/zcrx: udmabuf backed areas")

-- 
Pavel Begunkov

Re: [PATCH liburing] examples/zcrx: don't forget to sync dmabuf

[Jens Axboe]

On 10/16/26 10:36 AM, Pavel Begunkov wrote:
> On 10/16/26 17:29, Jens Axboe wrote:
>> On 10/16/26 10:25 AM, Pavel Begunkov wrote:
>>> dma-bufs are not always coherent and we need to sync it before
>>> accessing. Use proper dma-buf API for that.
>>
>> Can we get a Fixes tag for this please? Other fixes from today
>> lacked that as well, please always add fixes tags.
> 
> There was no "fixes" today.

Sure let's argue that, I merged 4 fixes from you this morning. None of
them had Fixes tags.

> liburing fixes tags either, but if you want to put one:
> 
> Fixes: 93d3a7a70b4a6 ("examples/zcrx: udmabuf backed areas")

Because it's always nice to have the reference, even if there are
no stable trees or anything like that on the liburing side.

-- 
Jens Axboe

Re: [PATCH liburing] examples/zcrx: don't forget to sync dmabuf

[Pavel Begunkov]

On 10/16/26 17:38, Jens Axboe wrote:
> On 10/16/26 10:36 AM, Pavel Begunkov wrote:
>> On 10/16/26 17:29, Jens Axboe wrote:
>>> On 10/16/26 10:25 AM, Pavel Begunkov wrote:
>>>> dma-bufs are not always coherent and we need to sync it before
>>>> accessing. Use proper dma-buf API for that.
>>>
>>> Can we get a Fixes tag for this please? Other fixes from today
>>> lacked that as well, please always add fixes tags.
>>
>> There was no "fixes" today.
> 
> Sure let's argue that, I merged 4 fixes from you this morning. None of
> them had Fixes tags.

There is nothing to argue about, I doubt anyone would call a
tabulation change or adding a kerneldoc "a fix".

>> liburing fixes tags either, but if you want to put one:
>>
>> Fixes: 93d3a7a70b4a6 ("examples/zcrx: udmabuf backed areas")
> 
> Because it's always nice to have the reference, even if there are
> no stable trees or anything like that on the liburing side.

Sure

-- 
Pavel Begunkov

Re: [PATCH liburing] examples/zcrx: don't forget to sync dmabuf

[Jens Axboe]

On 10/16/26 10:47 AM, Pavel Begunkov wrote:
> On 10/16/26 17:38, Jens Axboe wrote:
>> On 10/16/26 10:36 AM, Pavel Begunkov wrote:
>>> On 10/16/26 17:29, Jens Axboe wrote:
>>>> On 10/16/26 10:25 AM, Pavel Begunkov wrote:
>>>>> dma-bufs are not always coherent and we need to sync it before
>>>>> accessing. Use proper dma-buf API for that.
>>>>
>>>> Can we get a Fixes tag for this please? Other fixes from today
>>>> lacked that as well, please always add fixes tags.
>>>
>>> There was no "fixes" today.
>>
>> Sure let's argue that, I merged 4 fixes from you this morning. None of
>> them had Fixes tags.
> 
> There is nothing to argue about, I doubt anyone would call a
> tabulation change or adding a kerneldoc "a fix".

But we're talking about it, now another round... And justifying it. The
way I see it, anything that fixes something in a previous patch is, by
definition, a fix for that patch and should be labeled as such. That's
usually not something that people question. It doesn't need to fix
something that's functionally broken to be labeled a fix. For the
latter, if it's broken in a user visible way, we'd add stable to that
patch as well. Or just send it manually too, if it's missing from the
patch. That's the definition I use as the maintainer of those areas, and
that's why I ask for them.

-- 
Jens Axboe

Re: [PATCH liburing] examples/zcrx: don't forget to sync dmabuf

[Pavel Begunkov]

On 10/16/26 17:57, Jens Axboe wrote:
> On 10/16/26 10:47 AM, Pavel Begunkov wrote:
>> On 10/16/26 17:38, Jens Axboe wrote:
>>> On 10/16/26 10:36 AM, Pavel Begunkov wrote:
>>>> On 10/16/26 17:29, Jens Axboe wrote:
>>>>> On 10/16/26 10:25 AM, Pavel Begunkov wrote:
>>>>>> dma-bufs are not always coherent and we need to sync it before
>>>>>> accessing. Use proper dma-buf API for that.
>>>>>
>>>>> Can we get a Fixes tag for this please? Other fixes from today
>>>>> lacked that as well, please always add fixes tags.
>>>>
>>>> There was no "fixes" today.
>>>
>>> Sure let's argue that, I merged 4 fixes from you this morning. None of
>>> them had Fixes tags.
>>
>> There is nothing to argue about, I doubt anyone would call a
>> tabulation change or adding a kerneldoc "a fix".
> 
> But we're talking about it, now another round... And justifying it. The
> way I see it, anything that fixes something in a previous patch is, by
> definition, a fix for that patch and should be labeled as such. That's
> usually not something that people question. It doesn't need to fix
> something that's functionally broken to be labeled a fix. For the
> latter, if it's broken in a user visible way, we'd add stable to that
> patch as well. Or just send it manually too, if it's missing from the
> patch. That's the definition I use as the maintainer of those areas, and
> that's why I ask for them.

Jens, thank you for the insight. It was confusing because the public
documentation refer to bugs, but since you use it differently, I'll make
sure all patches that correct typos and indentation, add new docs,
introduce new APIs and extending existing ones, or do other enhancements
have a "Fixes" tag for you as the maintainer of those areas.

-- 
Pavel Begunkov

Re: [PATCH liburing] examples/zcrx: don't forget to sync dmabuf

[Jens Axboe]

On Fri, 16 Oct 2026 17:25:05 +0100, Pavel Begunkov wrote:
> dma-bufs are not always coherent and we need to sync it before
> accessing. Use proper dma-buf API for that.

Applied, thanks!

[1/1] examples/zcrx: don't forget to sync dmabuf
      commit: efa5f5efbeef6967d0a69e3b2c620650e76f4624

Best regards,
-- 
Jens Axboe