Re: [PATCH v2 1/4] soc: fsl: qmc: Only set completion interrupt when needed

[Christophe Leroy]

Hi Hervé,

Le 14/08/2025 à 09:34, Herve Codina a écrit :

Hi Christophe,

On Wed, 13 Aug 2025 12:06:51 +0200
Herve Codina <herve.cod...@bootlin.com> wrote:

Hi Christophe,

On Tue, 12 Aug 2025 12:50:55 +0200
Christophe Leroy <christophe.le...@csgroup.eu> wrote:

When no post-completion processing is expected, don't waste time
handling useless interrupts.

Only set QMC_BD_[R/T]X_I when a completion function is passed in,
and perform seamless completion on submit for interruptless buffers.

Signed-off-by: Christophe Leroy <christophe.le...@csgroup.eu>
---
v2: Keep the UB flag to mark not completed buffers and seamlessly flag them as 
completed during next submit.
---
  drivers/soc/fsl/qe/qmc.c | 44 ++++++++++++++++++++++++++++++----------
  1 file changed, 33 insertions(+), 11 deletions(-)

diff --git a/drivers/soc/fsl/qe/qmc.c b/drivers/soc/fsl/qe/qmc.c
index 36c0ccc06151f..8f76b9a5e385d 100644
--- a/drivers/soc/fsl/qe/qmc.c
+++ b/drivers/soc/fsl/qe/qmc.c
@@ -461,9 +461,16 @@ int qmc_chan_write_submit(struct qmc_chan *chan, 
dma_addr_t addr, size_t length,

ctrl = qmc_read16(&bd->cbd_sc);

        if (ctrl & (QMC_BD_TX_R | QMC_BD_TX_UB)) {
-               /* We are full ... */
-               ret = -EBUSY;
-               goto end;
+               if (!(ctrl & QMC_BD_TX_I) && bd == chan->txbd_done) {
+                       if (ctrl & QMC_BD_TX_W)
+                               chan->txbd_done = chan->txbds;
+                       else
+                               chan->txbd_done++;
+               } else {
+                       /* We are full ... */
+                       ret = -EBUSY;
+                       goto end;
+               }
        }

qmc_write16(&bd->cbd_datlen, length);

@@ -475,6 +482,10 @@ int qmc_chan_write_submit(struct qmc_chan *chan, 
dma_addr_t addr, size_t length,

/* Activate the descriptor */

        ctrl |= (QMC_BD_TX_R | QMC_BD_TX_UB);
+       if (complete)
+               ctrl |= QMC_BD_TX_I;
+       else
+               ctrl &= ~QMC_BD_TX_I;
        wmb(); /* Be sure to flush the descriptor before control update */
        qmc_write16(&bd->cbd_sc, ctrl);

You try to purge one descriptor for which the transfer is done but you do that
when you have no more free descriptors.

You end up with all descriptor "used". I think a better way to do that is
to purge all "done" descriptor configured to work without interrupts until a
descriptor with interrupt is found.

I have looked again at your code and looking for a free descriptor only when it
is needed is sufficient. You can forget my previous proposal.

Back to your code, I think you need to be sure that the descriptor you want to
re-use is really available and so you need to check the 'R' bit to be sure
that we are not with 'R' = 1 and 'UB' = 1 which means "BD is used, waiting for
a transfer".

Fixed in v3.

Thanks
Christophe