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SubjectRe: [PATCH] video: fbdev: pm2fb: avoid stall on fb_sync
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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@gmail.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

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