[Mesa-dev] [Bug 93686] Performance improvement ?=:=?UTF-8?Q? Please consider hardware ɢᴘᴜ rendering in llvmpipe

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=93686

--- Comment #24 from Alex Deucher <alexdeucher at gmail.com> ---
(In reply to ytrezq from comment #15)
> (In reply to Alex Deucher from comment #13)
> > I can't speak for intel, but on AMD APUs, while the GPU appears as a device
> > on the PCIE bus, it actually has a much faster internal connection to the
> > memory controller.
> You’re still confusing things. Of course they use the same memory controller
> directly. Of course they share the same memory modules.
> However they can’t read or write in memory of each others. So this behave
> like an external card ᴘᴄɪe card with it’s own memory modules (if we forget
> the bandwidth is also shared with an another device so each ones slow each
> others).
> 
> So if you want to send or receive data it can only happens over the ᴘᴄɪe
> bus, triggering the same synchronisations problems of external chipsets due
> to the bus bandwidth and instructions overhead.
> 
> Unified memory will only happen in future generations of graphics cards, but
> only behind a ᴘᴄɪe bus (which will slow things because there’s still the
> memory controller bus adding overhead), so we’re far from the time were ɢᴘᴜs
> of ᴀᴘᴜs will be able to access ʀᴀᴍ of ᴄᴘᴜs with the same overhead (at that
> time it’s expected ʀᴀᴍ modules would have been merged into the ᴄᴘᴜ chip
> meaning there would be no longer separated ʀᴀᴍ modules).

No, it happens right now.  The stolen memory used for APU "vram" is mainly for
vbios splash screen post messages to minimize the amount of gpu setup required
in the vbios and to provide contiguous memory which is slightly faster than
going through an MMU.  Once the APU driver has initialized it can map system
memory directly via the GPU's MMU.  Access to that memory does not go over the
pcie bus.  The GPU has a direct internal link the system memory similar to what
the CPU has.

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