linux-kernel - Re: [PATCH 1/5] ASoC: sh: rz-ssi: Drop calling rz_ssi_pio

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Message-ID: <CA+V-a8vz25B=cw_C4YMBRdDxeq7mi8Zc+noqpdHqfMP8eNHYFg@mail.gmail.com>
Date:   Mon, 10 Jan 2022 16:03:17 +0000
From:   "Lad, Prabhakar" <prabhakar.csengg@...il.com>
To:     Cezary Rojewski <cezary.rojewski@...el.com>,
        Biju Das <biju.das.jz@...renesas.com>,
        Pavel Machek <pavel@...x.de>
Cc:     Lad Prabhakar <prabhakar.mahadev-lad.rj@...renesas.com>,
        Liam Girdwood <lgirdwood@...il.com>,
        Mark Brown <broonie@...nel.org>,
        Jaroslav Kysela <perex@...ex.cz>,
        Takashi Iwai <tiwai@...e.com>,
        Linux-Renesas <linux-renesas-soc@...r.kernel.org>,
        alsa-devel <alsa-devel@...a-project.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/5] ASoC: sh: rz-ssi: Drop calling rz_ssi_pio_recv() recursively

Hi Cezary,

Thank you for the review.

On Mon, Jan 10, 2022 at 3:48 PM Cezary Rojewski
<cezary.rojewski@...el.com> wrote:
>
> On 2022-01-10 10:47 AM, Lad Prabhakar wrote:
> > Instead of recursively calling rz_ssi_pio_recv() use a while loop
> > to read the samples from RX fifo.
>
> Recursion and loops are means for doing something repeatedly. Could you
> specify _why_ such change was made i.e. the conversion from one method
> into the other? I bet the code is not being changed for the sake of
> changing it, the reason is simply missing in the commit message.
>
I had feedback from Pavel "recursion is unwelcome in kernel due to
limited stack use." which I did agree with as a result I have come up
with this patch. Also to add this driver will later be used on Renesas
RZ/A2 SoC's which runs with limited memory.

> Please note that refactoring below function into while-loop has a side
> effect: everything had to be indented by additional tab. Generally,
> readability increases if function is shaped 'linearly'.
>
I do agree, my initial patch just added a jump back to the start of
the function if there are more samples, but Biju suggested to use a
while loop instead.

> > This also fixes an issue where the return value of rz_ssi_pio_recv()
> > was ignored when called recursively.
> >
> > Fixes: 03e786bd4341 ("ASoC: sh: Add RZ/G2L SSIF-2 driver")
> > Reported-by: Pavel Machek <pavel@...x.de>
> > Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@...renesas.com>
> > Reviewed-by: Biju Das <biju.das.jz@...renesas.com>
> > ---
> >   sound/soc/sh/rz-ssi.c | 68 ++++++++++++++++++++++---------------------
> >   1 file changed, 35 insertions(+), 33 deletions(-)
> >
> > diff --git a/sound/soc/sh/rz-ssi.c b/sound/soc/sh/rz-ssi.c
> > index fa0cc08f70ec..37466f65c2b0 100644
> > --- a/sound/soc/sh/rz-ssi.c
> > +++ b/sound/soc/sh/rz-ssi.c
> > @@ -411,54 +411,56 @@ static int rz_ssi_pio_recv(struct rz_ssi_priv *ssi, struct rz_ssi_stream *strm)
> >   {
> >       struct snd_pcm_substream *substream = strm->substream;
> >       struct snd_pcm_runtime *runtime;
> > +     bool done = false;
> >       u16 *buf;
> >       int fifo_samples;
> >       int frames_left;
> > -     int samples = 0;
> > +     int samples;
> >       int i;
> >
> >       if (!rz_ssi_stream_is_valid(ssi, strm))
> >               return -EINVAL;
> >
> >       runtime = substream->runtime;
> > -     /* frames left in this period */
> > -     frames_left = runtime->period_size - (strm->buffer_pos %
> > -                                           runtime->period_size);
> > -     if (frames_left == 0)
> > -             frames_left = runtime->period_size;
> >
> > -     /* Samples in RX FIFO */
> > -     fifo_samples = (rz_ssi_reg_readl(ssi, SSIFSR) >>
> > -                     SSIFSR_RDC_SHIFT) & SSIFSR_RDC_MASK;
> > -
> > -     /* Only read full frames at a time */
> > -     while (frames_left && (fifo_samples >= runtime->channels)) {
> > -             samples += runtime->channels;
> > -             fifo_samples -= runtime->channels;
> > -             frames_left--;
> > -     }
> > +     while (!done) {
>
> I wonder if converting this into do-while isn't a better option. Maybe
> I'm missing something but 'done' flag seems to be changed only as an
> outcome of the last if-statement (last step) in this entire procedure.
> Perhaps condition from said if-statement could also be moved into
> 'while' portion of do-while loop.
>
Agreed.

> > +             /* frames left in this period */
> > +             frames_left = runtime->period_size -
> > +                           (strm->buffer_pos % runtime->period_size);
> > +             if (!frames_left)
> > +                     frames_left = runtime->period_size;
> > +
> > +             /* Samples in RX FIFO */
> > +             fifo_samples = (rz_ssi_reg_readl(ssi, SSIFSR) >>
> > +                             SSIFSR_RDC_SHIFT) & SSIFSR_RDC_MASK;
> > +
> > +             /* Only read full frames at a time */
> > +             samples = 0;
> > +             while (frames_left && (fifo_samples >= runtime->channels)) {
> > +                     samples += runtime->channels;
> > +                     fifo_samples -= runtime->channels;
> > +                     frames_left--;
> > +             }
> >
> > -     /* not enough samples yet */
> > -     if (samples == 0)
> > -             return 0;
> > +             /* not enough samples yet */
> > +             if (!samples)
> > +                     break;
> >
> > -     /* calculate new buffer index */
> > -     buf = (u16 *)(runtime->dma_area);
> > -     buf += strm->buffer_pos * runtime->channels;
> > +             /* calculate new buffer index */
> > +             buf = (u16 *)(runtime->dma_area);
>
> Is the second pair of brackets needed?
>
Nope can be dropped.

Cheers,
Prabhakar