linux-kernel - Re: [PATCHv5,RESEND 8/8] drm: tegra: Add gr2d device

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Date:	Sun, 10 Feb 2013 16:42:53 -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 8/8] drm: tegra: Add gr2d device

On 07.02.2013 23:07, Thierry Reding wrote:
> On Wed, Feb 06, 2013 at 01:23:17PM -0800, Terje Bergström wrote:
>>>> That's the security firewall. It walks through each submit, and ensures
>>>> that each register write that writes an address, goes through the host1x
>>>> reloc mechanism. This way user space cannot ask 2D to write to arbitrary
>>>> memory locations.
>>> I see. Can this be made more generic? Perhaps adding a table of valid
>>> registers to the device and use a generic function to iterate over that
>>> instead of having to provide the same function for each client.
>> For which one does gcc generate more efficient code? I've thought a
>> switch-case statement might get compiled into something more efficient
>> than a table lookup.
>> But the rest of the code is generic - just the one function which
>> compares against known address registers is specific to 2D.
> Table lookup should be pretty fast. I wouldn't worry too much about
> performance at this stage, though. Readability is more important in my
> opinion. A lookup table is a lot more readable and reusable I think. If
> it turns out that using a function is actually faster we can always
> optimize later.

You're right about performance. We already saw quite a bad performance
hit with the current firewall, so we'll need to worry about performance
later.

I'll take a look at converting the register list to a table. Instead of
always doing a linear search of a table, a bitfield might be more
appropriate.

Terje
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