linux-kernel - Re: [PATCH 02/12] drm/nouveau/timer: skip calibration on GK20A

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Message-ID: <20140324215408.GB17218@mithrandir>
Date:	Mon, 24 Mar 2014 22:54:09 +0100
From:	Thierry Reding <thierry.reding@...il.com>
To:	Alexandre Courbot <acourbot@...dia.com>
Cc:	Ben Skeggs <bskeggs@...hat.com>, nouveau@...ts.freedesktop.org,
	dri-devel@...ts.freedesktop.org, linux-tegra@...r.kernel.org,
	linux-kernel@...r.kernel.org, gnurou@...il.com
Subject: Re: [PATCH 02/12] drm/nouveau/timer: skip calibration on GK20A

On Mon, Mar 24, 2014 at 05:42:24PM +0900, Alexandre Courbot wrote:
> GK20A's timer is directly attached to the system timer and cannot be
> calibrated. Skip the calibration phase on that chip since the
> corresponding registers do not exist.
> 
> Signed-off-by: Alexandre Courbot <acourbot@...dia.com>
> ---
>  drivers/gpu/drm/nouveau/core/subdev/timer/nv04.c | 19 +++++++++++++------
>  1 file changed, 13 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/gpu/drm/nouveau/core/subdev/timer/nv04.c b/drivers/gpu/drm/nouveau/core/subdev/timer/nv04.c
> index c0bdd10358d7..822fe0d8a871 100644
> --- a/drivers/gpu/drm/nouveau/core/subdev/timer/nv04.c
> +++ b/drivers/gpu/drm/nouveau/core/subdev/timer/nv04.c
> @@ -185,6 +185,10 @@ nv04_timer_init(struct nouveau_object *object)
>  	if (ret)
>  		return ret;
>  
> +	/* gk20a does not have the calibration registers */
> +	if (device->chipset == 0xea)
> +		goto skip_clk_init;

I'm concerned that this won't scale in the future. Perhaps a better
solution would be to add a "flags" or "features" field to struct
nouveau_device along with feature bits such as HAS_CALIBRATION or
similar.

That way we don't have to touch this code for every new future Tegra
chip. Unless perhaps if there's a reason to expect things to change in
newer generations.

Thierry

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