Re: [PATCH RFC 09/15] drm: imx: Add MIPI DSI host controller driver

[Liu Ying]

Hi Russell,

On 12/17/2014 06:40 PM, Russell King - ARM Linux wrote:
> On Wed, Dec 17, 2014 at 05:44:33PM +0800, Liu Ying wrote:
> > Hi Thierry,
> >
> > Sorry for the late response.
> > I tried to address almost all your comments locally first.
> > More feedback below.
> >
> > On 12/10/2014 09:16 PM, Thierry Reding wrote:
> > > On Wed, Dec 10, 2014 at 04:37:22PM +0800, Liu Ying wrote:
> > > > +static int check_status(struct imx_mipi_dsi *dsi, u32 reg, u32 status,
> > > > +			int timeout, bool to_set)
> > > > +{
> > > > +	u32 val;
> > > > +	bool out = false;
> > > > +
> > > > +	val = dsi_read(dsi, reg);
> > > > +	for (;;) {
> > > > +		out = to_set ? (val & status) : !(val & status);
> > > > +		if (out)
> > > > +			break;
> > > > +
> > > > +		if (!timeout--)
> > > > +			return -EFAULT;
> > > > +
> > > > +		msleep(1);
> > > > +		val = dsi_read(dsi, reg);
> > > > +	}
> > > > +	return 0;
> > > > +}
> > >
> > > You should probably use a properly timed loop here. msleep() isn't
> > > guaranteed to return after exactly one millisecond, so your timeout is
> > > never going to be accurate. Something like the following would be better
> > > in my opinion:
> > >
> > > 	timeout = jiffies + msecs_to_jiffies(timeout);
> > >
> > > 	while (time_before(jiffies, timeout)) {
> > > 		...
> > > 	}
> > >
> > > Also timeout should be unsigned long in that case.
> >
> > Accepted.
>
> Actually, that's a bad example: what we want to do is to assess success
> after we wait, before we decide that something has failed.  In other
> words, we don't want to wait, and decide that we failed without first
> checking for success.
>
> In any case, returning -EFAULT is not sane: EFAULT doesn't mean "fault"
> it means "Bad address", and it is returned to userspace to mean that
> userspace passed the kernel a bad address.  That definition does /not/
> fit what's going on here.
>
> 	timeout = jiffies + msecs_to_jiffies(timeout);
>
> 	do {
> 		val = dsi_read(dsi, reg);
> 		out = to_set ? (val & status) : !(val & status);
> 		if (out)
> 			break;
>
> 		if (time_is_after_jiffies(timeout))

time_is_after_jiffies(a) is defined as time_before(jiffies, a).

So, this line should be changed to

	if (time_after(jiffies, timeout))

Right?

> 			return -ETIMEDOUT;
>
> 		msleep(1);
> 	} while (1);
>
> 	return 0;
>
> would be better: we only fail immediately after we have checked whether
> we succeeded, and we also do the first check immediately.

Does this one look better?  I use cpu_relax() instead of msleep(1).

        expire = jiffies + msecs_to_jiffies(timeout);
        for (;;) {
                val = dsi_read(dsi, reg);
                out = to_set ? (val & status) : !(val & status);
                if (out)
                        break;

                if (time_after(jiffies, expire))
                        return -ETIMEDOUT;

                cpu_relax();
        }

	return 0;

Regards,

Liu Ying
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/