linux-kernel - Re: [PATCH] video: fbdev: pm2fb: avoid stall on fb

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Message-ID: <CAA5qM4D0y9-gL7AAMQvtwrfJC_UvSTB5_S5PX3qkYSxHRUS-Qw@mail.gmail.com>
Date:   Sat, 20 Feb 2021 19:00:31 -0500
From:   Tong Zhang <ztong0001@...il.com>
To:     Randy Dunlap <rdunlap@...radead.org>
Cc:     "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        Sam Ravnborg <sam@...nborg.org>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
        dri-devel@...ts.freedesktop.org, linux-fbdev@...r.kernel.org,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] video: fbdev: pm2fb: avoid stall on fb_sync

Hi Randy,
Thanks for the comment.
I currently have this problem on my machine.
I have submitted a revised patch -- which includes the console log.
Thanks!
- Tong

On Sat, Feb 20, 2021 at 6:33 PM Randy Dunlap <rdunlap@...radead.org> wrote:
>
> Hi--
>
> On 2/20/21 3:02 PM, Tong Zhang wrote:
> > pm2fb_sync is called when doing /dev/fb read or write.
> > The original pm2fb_sync wait indefinitely on hardware flags which can
> > possibly stall kernel and make everything unresponsive.
> > Instead of waiting indefinitely, we can timeout to give user a chance to
> > get back control.
>
> Is this a real problem or theoretical?
> Does someone still use this driver?
>
>
> > Signed-off-by: Tong Zhang <ztong0001@...il.com>
> > ---
> >  drivers/video/fbdev/pm2fb.c | 29 ++++++++++++++++++++++++++---
> >  1 file changed, 26 insertions(+), 3 deletions(-)
> >
> > diff --git a/drivers/video/fbdev/pm2fb.c b/drivers/video/fbdev/pm2fb.c
> > index 27893fa139b0..8578c64a0c54 100644
> > --- a/drivers/video/fbdev/pm2fb.c
> > +++ b/drivers/video/fbdev/pm2fb.c
> > @@ -183,12 +183,23 @@ static inline void pm2v_RDAC_WR(struct pm2fb_par *p, s32 idx, u32 v)
> >
> >  #ifdef CONFIG_FB_PM2_FIFO_DISCONNECT
> >  #define WAIT_FIFO(p, a)
> > +#define WAIT_FIFO_TIMEOUT(p, a) (0)
> >  #else
> >  static inline void WAIT_FIFO(struct pm2fb_par *p, u32 a)
> >  {
> >       while (pm2_RD(p, PM2R_IN_FIFO_SPACE) < a)
> >               cpu_relax();
> >  }
> > +static int inline void WAIT_FIFO_TIMEOUT(struct pm2fb_par *p, u32 a)
>
> drop      void       ^^^
> It's already "int".
> Did you compile this?
>
> > +{
> > +     int timeout = 10000;
> > +     while (pm2_RD(p, PM2R_IN_FIFO_SPACE) < a) {
> > +             cpu_relax();
> > +             if (--timeout==0)
>
> spaces around ==
>
> > +                     return 1;
> > +     }
> > +     return 0;
> > +}
> >  #endif
> >
> >  /*
> > @@ -1031,15 +1042,27 @@ static int pm2fb_blank(int blank_mode, struct fb_info *info)
> >  static int pm2fb_sync(struct fb_info *info)
> >  {
> >       struct pm2fb_par *par = info->par;
> > +     int timeout_sync = 10000;
> > +     int timeout_fifo;
> >
> > -     WAIT_FIFO(par, 1);
> > +     if (WAIT_FIFO_TIMEOUT(par, 1))
> > +             goto end;
> >       pm2_WR(par, PM2R_SYNC, 0);
> >       mb();
> >       do {
> > -             while (pm2_RD(par, PM2R_OUT_FIFO_WORDS) == 0)
> > +             timeout_fifo = 10000;
> > +             while (pm2_RD(par, PM2R_OUT_FIFO_WORDS) == 0) {
> >                       cpu_relax();
> > -     } while (pm2_RD(par, PM2R_OUT_FIFO) != PM2TAG(PM2R_SYNC));
> > +                     if (--timeout_fifo==0)
>
> spaces around ==
>
> > +                             goto end;
> > +             }
> > +             if (pm2_RD(par, PM2R_OUT_FIFO) == PM2TAG(PM2R_SYNC))
> > +                     break;
> > +     } while (--timeout_sync>0);
>
> spaces around >
>
> >
> > +end:
> > +     if ((!timeout_sync) || (!timeout_fifo))
> > +             printk_ratelimited(KERN_WARNING "pm2fb: sync timeout!\n");
> >       return 0;
> >  }
> >
> >
>
>
> thanks.
> --
> ~Randy