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Message-ID: <20250813120651.27dc8467@bootlin.com>
Date: Wed, 13 Aug 2025 12:06:51 +0200
From: Herve Codina <herve.codina@...tlin.com>
To: Christophe Leroy <christophe.leroy@...roup.eu>
Cc: Qiang Zhao <qiang.zhao@....com>, Shengjiu Wang
<shengjiu.wang@...il.com>, Xiubo Li <Xiubo.Lee@...il.com>, Fabio Estevam
<festevam@...il.com>, Nicolin Chen <nicoleotsuka@...il.com>, Liam Girdwood
<lgirdwood@...il.com>, Mark Brown <broonie@...nel.org>, Jaroslav Kysela
<perex@...ex.cz>, Takashi Iwai <tiwai@...e.com>,
linuxppc-dev@...ts.ozlabs.org, linux-arm-kernel@...ts.infradead.org,
linux-kernel@...r.kernel.org, linux-sound@...r.kernel.org
Subject: Re: [PATCH v2 1/4] soc: fsl: qmc: Only set completion interrupt
when needed
Hi Christophe,
On Tue, 12 Aug 2025 12:50:55 +0200
Christophe Leroy <christophe.leroy@...roup.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@...roup.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.
What do you think about the following (draft code not compiled and not tested) ?
/* must be called with tx_lock taken */
static void qmc_chan_write_purge(struct qmc_chan *chan)
{
cbd_t __iomem *bd;
u16 ctrl;
/*
* Purge descriptors configured to work without interrupts
* (I bit == 0) those descriptors have no completion
* callbacks.
*
* R bit UB bit
* 0 0 : The BD is free
* 1 1 : The BD is in used, waiting for transfer
* 0 1 : The BD is in used, waiting for completion
* 1 0 : Should not append
*
* We purge those descriptors which are in the state "waiting for
* completion" up to the first one configured to work with an interrupt.
*
*/
bd = chan->txbd_done;
ctrl = qmc_read16(&bd->cbd_sc);
while (!(ctrl & (QMC_BD_TX_R | QMC_BD_TX_I)) {
if (!(ctrl & QMC_BD_TX_UB))
return;
qmc_write16(&bd->cbd_sc, ctrl & ~QMC_BD_TX_UB);
if (ctrl & QMC_BD_TX_W)
chan->txbd_done = chan->txbds;
else
chan->txbd_done++;
bd = chan->txbd_done;
ctrl = qmc_read16(&bd->cbd_sc);
}
}
Then, qmc_chan_write_submit() calls the purge function as a first operation.
This will look to something like the following:
int qmc_chan_write_submit(struct qmc_chan *chan, dma_addr_t addr, size_t length,
void (*complete)(void *context), void *context)
{
struct qmc_xfer_desc *xfer_desc;
unsigned long flags;
cbd_t __iomem *bd;
u16 ctrl;
int ret;
/*
* R bit UB bit
* 0 0 : The BD is free
* 1 1 : The BD is in used, waiting for transfer
* 0 1 : The BD is in used, waiting for completion
* 1 0 : Should not append
*/
spin_lock_irqsave(&chan->tx_lock, flags);
qmc_chan_write_purge(chan);
bd = chan->txbd_free;
...
if (complete)
ctrl |= QMC_BD_TX_I;
else
ctrl &= ~QMC_BD_TX_I;
...
}
> @@ -569,9 +580,16 @@ int qmc_chan_read_submit(struct qmc_chan *chan, dma_addr_t addr, size_t length,
>
...
Exact same comment and proposal for read part.
>
> @@ -1482,19 +1504,19 @@ static int qmc_setup_chan(struct qmc *qmc, struct qmc_chan *chan)
>
> /* Init Rx BDs and set Wrap bit on last descriptor */
> BUILD_BUG_ON(QMC_NB_RXBDS == 0);
> - val = QMC_BD_RX_I;
> for (i = 0; i < QMC_NB_RXBDS; i++) {
> bd = chan->rxbds + i;
> - qmc_write16(&bd->cbd_sc, val);
> + qmc_write16(&bd->cbd_sc, 0);
> }
> bd = chan->rxbds + QMC_NB_RXBDS - 1;
> - qmc_write16(&bd->cbd_sc, val | QMC_BD_RX_W);
> + qmc_write16(&bd->cbd_sc, QMC_BD_RX_W);
>
> /* Init Tx BDs and set Wrap bit on last descriptor */
> BUILD_BUG_ON(QMC_NB_TXBDS == 0);
> - val = QMC_BD_TX_I;
> if (chan->mode == QMC_HDLC)
> - val |= QMC_BD_TX_L | QMC_BD_TX_TC;
> + val = QMC_BD_TX_L | QMC_BD_TX_TC;
> + else
> + val = 0;
> for (i = 0; i < QMC_NB_TXBDS; i++) {
> bd = chan->txbds + i;
> qmc_write16(&bd->cbd_sc, val);
Ok for modifications in qmc_setup_chan().
Best regards,
Hervé
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