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

[Herve Codina <herve.codina@bootlin.com>]

Hi Christophe,

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

> Hi Christophe,
> 
> On Tue, 12 Aug 2025 12:50:55 +0200
> Christophe Leroy <christophe.leroy@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.leroy@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".

For instance:

	if (ctrl & (QMC_BD_TX_R | QMC_BD_TX_UB)) {
		if (!(ctrl & (QMC_BD_TX_I | QMC_BD_TX_R) &&
		    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;
		}
	}

Best regards,
Hervé