linux-kernel - Re: [PATCH] clk: bcm: rpi: Mark ISP clock as critical

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Message-ID: <176232548379.1167490.1126479868042439951@freya>
Date: Wed, 05 Nov 2025 12:21:23 +0530
From: Jai Luthra <jai.luthra@...asonboard.com>
To: Broadcom internal kernel review list <bcm-kernel-feedback-list@...adcom.com>, Florian Fainelli <florian.fainelli@...adcom.com>, Maíra Canal <mcanal@...lia.com>, Michael Turquette <mturquette@...libre.com>, Stefan Wahren <wahrenst@....net>, Stephen Boyd <sboyd@...nel.org>
Cc: linux-clk@...r.kernel.org, linux-rpi-kernel@...ts.infradead.org, linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] clk: bcm: rpi: Mark ISP clock as critical

Hi Florian,

Quoting Florian Fainelli (2025-11-05 03:50:23)
> On 10/28/25 04:46, Jai Luthra wrote:
> > The ISP clock should also be marked critical to prevent the clock
> > framework from disabling it on boot, which can lead to bus lockups when
> > the kernel tries to submit buffers to the VC4 ISP firmware.
> 
> The commit message suggests that the driver that interfaces with the VC4 
> ISP firmware is not ensuring that the clock remains enabled for the 
> duration of the transactions. That would be a driver bug, would not it?
> 

Yes, from https://github.com/raspberrypi/linux/issues/7100#issuecomment-3467699055
the VC4 firmware should be responsible for stopping and starting that clock
when necessary.

The downstream bcm2835-isp linux driver currently does not consume this
clock at all, so I'm not sure if the cleaner solution is to drop this
altogether from linux, or mark it as a consumer, along with fixing the
firmware driver.

> > 
> > Link: https://github.com/raspberrypi/linux/issues/7100
> > Fixes: 919d6924ae9b ("clk: bcm: rpi: Turn firmware clock on/off when preparing/unpreparing")
> > Signed-off-by: Jai Luthra <jai.luthra@...asonboard.com>
> > ---
> >   drivers/clk/bcm/clk-raspberrypi.c | 1 +
> >   1 file changed, 1 insertion(+)
> > 
> > diff --git a/drivers/clk/bcm/clk-raspberrypi.c b/drivers/clk/bcm/clk-raspberrypi.c
> > index 1a9162f0ae31e330c46f6eafdd00350599b0eede..dcc3ae5fb91650b042d52f02499922786b43afde 100644
> > --- a/drivers/clk/bcm/clk-raspberrypi.c
> > +++ b/drivers/clk/bcm/clk-raspberrypi.c
> > @@ -151,6 +151,7 @@ raspberrypi_clk_variants[RPI_FIRMWARE_NUM_CLK_ID] = {
> >       [RPI_FIRMWARE_ISP_CLK_ID] = {
> >               .export = true,
> >               .minimize = true,
> > +             .flags = CLK_IS_CRITICAL,
> >       },
> >       [RPI_FIRMWARE_PIXEL_BVB_CLK_ID] = {
> >               .export = true,
> > 
> > ---
> > base-commit: 3a8660878839faadb4f1a6dd72c3179c1df56787
> > change-id: 20251028-rpi-isp-clk-eb5c48346e51
> > 
> > Best regards,
> 
> 
> -- 
> Florian

Thanks,
Jai