linux-kernel - Re: [PATCHv5,RESEND 1/8] gpu: host1x: Add host1x driver

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Date:	Wed, 6 Feb 2013 12:13:40 -0800
From:	Terje Bergström <tbergstrom@...dia.com>
To:	Thierry Reding <thierry.reding@...onic-design.de>
CC:	Arto Merilainen <amerilainen@...dia.com>,
	"airlied@...ux.ie" <airlied@...ux.ie>,
	"dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
	"linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCHv5,RESEND 1/8] gpu: host1x: Add host1x driver

On 04.02.2013 23:43, Thierry Reding wrote:
> My point was that you could include the call to host1x_syncpt_reset()
> within host1x_syncpt_init(). That will keep unneeded code out of the
> host1x_probe() function. Also you don't want to use the syncpoints
> uninitialized, right?

Of course, sorry, I misunderstood. That makes a lot of sense.

>>>> + */
>>>> +static u32 syncpt_load_min(struct host1x_syncpt *sp)
>>>> +{
>>>> +     struct host1x *dev = sp->dev;
>>>> +     u32 old, live;
>>>> +
>>>> +     do {
>>>> +             old = host1x_syncpt_read_min(sp);
>>>> +             live = host1x_sync_readl(dev,
>>>> +                             HOST1X_SYNC_SYNCPT_0 + sp->id * 4);
>>>> +     } while ((u32)atomic_cmpxchg(&sp->min_val, old, live) != old);
>>>
>>> I think this warrants a comment.
>>
>> Sure. It just loops in case there's a race writing to min_val.
> 
> Oh, I see. That'd make a good comment. Is the cast to (u32) really
> necessary?

I'll add a comment. atomic_cmpxchg returns a signed value, so I think
the cast is needed.

> Save/restore has the disadvantage of the direction not being implicit.
> Save could mean save to hardware or save to software. The same is true
> for restore. However if the direction is clearly defined, save and
> restore work for me.
> 
> Maybe the comment could be changed to be more explicit. Something like:
> 
> 	/*
> 	 * Write cached syncpoint and waitbase values to hardware.
> 	 */
> 
> And for host1x_syncpt_save():
> 
> 	/*
> 	 * For client-managed registers, update the cached syncpoint and
> 	 * waitbase values by reading from the registers.
> 	 */

I was using save in the same way as f.ex. i915 (i915_suspend.c): save
state of hardware to RAM, restore state from RAM. I'll add proper
comments, but save and restore are for all syncpts, not only client managed.

> 
>>>> +/*
>>>> + * Updates the last value read from hardware.
>>>> + */
>>>> +u32 host1x_syncpt_load_min(struct host1x_syncpt *sp)
>>>> +{
>>>> +     u32 val;
>>>> +     val = sp->dev->syncpt_op.load_min(sp);
>>>> +     trace_host1x_syncpt_load_min(sp->id, val);
>>>> +
>>>> +     return val;
>>>> +}
> Maybe the function should be called host1x_syncpt_load() if there is no
> equivalent way to load the maximum value (since there is no register to
> read from).

Sounds good. Maximum is just a software concept.

> That's certainly true for interrupts. However, if you look at the DMA
> subsystem for example, you can also request an unnamed resource.
> 
> The difference is sufficiently subtle that host1x_syncpt_allocate()
> would work for me too, though. I just have a slight preference for
> host1x_syncpt_request().

I don't really have a strong preference, so I'll follow your suggestion.

Terje
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