linux-kernel - RE: [PATCH] video: hyperv_fb: Allow resolutions with size

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Message-ID: <BYAPR21MB1270E4CBA78869748D28C81BBFD19@BYAPR21MB1270.namprd21.prod.outlook.com>
Date:   Wed, 18 May 2022 18:48:39 +0000
From:   Dexuan Cui <decui@...rosoft.com>
To:     Dexuan Cui <decui@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Wei Liu <wei.liu@...nel.org>,
        Saurabh Sengar <ssengar@...ux.microsoft.com>,
        "Michael Kelley (LINUX)" <mikelley@...rosoft.com>
CC:     Saurabh Singh Sengar <ssengar@...rosoft.com>,
        KY Srinivasan <kys@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        "deller@....de" <deller@....de>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
        "linux-fbdev@...r.kernel.org" <linux-fbdev@...r.kernel.org>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] video: hyperv_fb: Allow resolutions with size > 64 MB for
 Gen1

> From: Dexuan Cui <decui@...rosoft.com>
> Sent: Wednesday, May 4, 2022 10:05 AM
> To: Haiyang Zhang <haiyangz@...rosoft.com>; Wei Liu <wei.liu@...nel.org>;
> > ...
> > When I initially implemented this driver 10 years ago, I believe there
> > was smaller limit for the fb... But I think this patch is good for the
> > newer MMIO alloc scheme. I hope to see reviews also from
> >  @Dexuan Cui @Michael Kelley (LINUX) who are more familiar with
> > the PCI/BAR/MMIO area.
> >
> > Thanks,
> > - Haiyang
> 
> The patch looks good to me but I suggest we check with the Hyper-V
> team to figure out how a Gen1 Windows VM supports a higher
> resolution that needs a VRAM size of more than 64MB. Just in case we
> miss something..
> 
> Thanks,
> -- Dexuan

Reviewed-by: Dexuan Cui <decui@...rosoft.com>

Saurabh checked this with Hyper-V team, who said there is no
Generation 1-specific block for larger VRAM sizes in Windows VM.

When the driver was originally developed, we didn't have the API
vmbus_allocate_mmio(), and I guess we just used the PCI device's BAR
address for simplicity, and didn't realize the restriction with very high
resolutions that require >64 MB VRAM. It looks like the synthetic
VMBus framebuffer device doesn't have to use the same MMIO range
used by the Hyper-V legacy PCI framebuffer device, so the patch
looks good to me.

BTW, please check the hyperv-drm driver as well:
drivers/gpu/drm/hyperv/hyperv_drm_drv.c
I think we should make the same change there to support 7680x4320
for Gen1 VMs.

Thanks,
Dexuan