linux-kernel - Re: [PATCH RFT v2 1/8] media: hantro: jpeg: Relax register writes before write starting hardware

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Message-ID: <CAAEAJfBM4d4hd1Av_TO-WVXQoUCNUckm+YHawdg6PY3urkB9nA@mail.gmail.com>
Date:   Wed, 19 Jan 2022 08:59:51 -0300
From:   Ezequiel Garcia <ezequiel@...guardiasur.com.ar>
To:     Chen-Yu Tsai <wenst@...omium.org>
Cc:     Hans Verkuil <hverkuil-cisco@...all.nl>,
        Philipp Zabel <p.zabel@...gutronix.de>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-media <linux-media@...r.kernel.org>,
        "open list:ARM/Rockchip SoC..." <linux-rockchip@...ts.infradead.org>,
        "open list:STAGING SUBSYSTEM" <linux-staging@...ts.linux.dev>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RFT v2 1/8] media: hantro: jpeg: Relax register writes
 before write starting hardware

On Wed, 19 Jan 2022 at 07:09, Chen-Yu Tsai <wenst@...omium.org> wrote:
>
> Hi,
>
> On Wed, Jan 19, 2022 at 5:02 AM Ezequiel Garcia
> <ezequiel@...guardiasur.com.ar> wrote:
> >
> > Hi Chen-Yu,
> >
> > The series looks good, thanks for picking up this task.
> >
> > Just a one comment.
> >
> > On Fri, Jan 07, 2022 at 05:34:48PM +0800, Chen-Yu Tsai wrote:
> > > In the earlier submissions of the Hantro/Rockchip JPEG encoder driver, a
> > > wmb() was inserted before the final register write that starts the
> > > encoder. In v11, it was removed and the second-to-last register write
> > > was changed to a non-relaxed write, which has an implicit wmb() [1].
> > > The rockchip_vpu2 (then rk3399_vpu) variant is even weirder as there
> > > is another writel_relaxed() following the non-relaxed one.
> > >
> > > Turns out only the last writel() needs to be non-relaxed. Device I/O
> > > mappings already guarantee strict ordering to the same endpoint, and
> > > the writel() triggering the hardware would force all writes to memory
> > > to be observed before the writel() to the hardware is observed.
> > >
> > > [1] https://lore.kernel.org/linux-media/CAAFQd5ArFG0hU6MgcyLd+_UOP3+T_U-aw2FXv6sE7fGqVCVGqw@mail.gmail.com/
> > >
> > > Signed-off-by: Chen-Yu Tsai <wenst@...omium.org>
> > > ---
> > >  drivers/staging/media/hantro/hantro_h1_jpeg_enc.c        | 3 +--
> > >  drivers/staging/media/hantro/rockchip_vpu2_hw_jpeg_enc.c | 3 +--
> > >  2 files changed, 2 insertions(+), 4 deletions(-)
> > >
> > > diff --git a/drivers/staging/media/hantro/hantro_h1_jpeg_enc.c b/drivers/staging/media/hantro/hantro_h1_jpeg_enc.c
> > > index 1450013d3685..03db1c3444f8 100644
> > > --- a/drivers/staging/media/hantro/hantro_h1_jpeg_enc.c
> > > +++ b/drivers/staging/media/hantro/hantro_h1_jpeg_enc.c
> > > @@ -123,8 +123,7 @@ int hantro_h1_jpeg_enc_run(struct hantro_ctx *ctx)
> > >               | H1_REG_AXI_CTRL_INPUT_SWAP32
> > >               | H1_REG_AXI_CTRL_OUTPUT_SWAP8
> > >               | H1_REG_AXI_CTRL_INPUT_SWAP8;
> > > -     /* Make sure that all registers are written at this point. */
> > > -     vepu_write(vpu, reg, H1_REG_AXI_CTRL);
> > > +     vepu_write_relaxed(vpu, reg, H1_REG_AXI_CTRL);
> > >
> >
> > As far as I can remember, this logic comes from really old Chromium Kernels.
> > You might be right, and this barrier isn't needed... but then OTOH the comment
> > is here for a reason, so maybe it is needed (or was needed on some RK3288 SoC revision).
>
> I just realized that my commit log is wrong.
>
> " ... a wmb() was inserted before the final register write that starts the
> encoder. ... " . It is actually before the second-to-last register write.
>
> > I don't have RK3288 boards near me, but in any case, I'm not sure
> > we'd be able to test this easily (maybe there are issues that only
> > trigger under a certain load).
>
> I see. I do have a Veyron around that I haven't used in awhile. But as you
> said, it might not be an obvious hardware limitation.
>
> > I'd personally avoid this one change, but if you are confident enough with it
> > that's fine too.
>
> Unfortunately they didn't leave a whole lot of clues around. For most hardware,
> as I mentioned in the commit log, I think the final non-relaxed write should
> suffice. I'd point to the decoder drivers not having any barriers or
> non-relaxed writes except the final one, but IIUC they are actually two
> distinct pieces of hardware.
>
> I suspect we will never know. This JPEG encoder doesn't seem to get used
> a lot. The VP8 and H.264 encoders on ChromeOS work correctly without the
> extra barrier and get tested a lot, but that's only testing the RK3399.
>
> Hans, would it be possible for you to skip this patch and pick the rest?
> Or would you like me to resent without this one?
>

If you decide to resend, feel free to add:

Reviewed-by: Ezequiel Garcia <ezequiel@...guardiasur.com.ar>

to the whole series.

Thanks,
Ezequiel