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

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Date:	Thu, 18 Dec 2014 10:46:33 +0800
From:	Liu Ying <Ying.Liu@...escale.com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
CC:	Thierry Reding <thierry.reding@...il.com>,
	<dri-devel@...ts.freedesktop.org>, <devicetree@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>,
	<linux-arm-kernel@...ts.infradead.org>, <p.zabel@...gutronix.de>,
	<shawn.guo@...aro.org>, <kernel@...gutronix.de>,
	<mturquette@...aro.org>, <airlied@...ux.ie>
Subject: Re: [PATCH RFC 09/15] drm: imx: Add MIPI DSI host controller driver

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
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