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From:   Dexuan Cui <decui@microsoft.com>
To:     Boqun Feng <boqun.feng@gmail.com>,
        "wei.liu@kernel.org" <wei.liu@kernel.org>
CC:     KY Srinivasan <kys@microsoft.com>,
        Haiyang Zhang <haiyangz@microsoft.com>,
        Stephen Hemminger <sthemmin@microsoft.com>,
        "b.zolnierkie@samsung.com" <b.zolnierkie@samsung.com>,
        "linux-hyperv@vger.kernel.org" <linux-hyperv@vger.kernel.org>,
        "dri-devel@lists.freedesktop.org" <dri-devel@lists.freedesktop.org>,
        "linux-fbdev@vger.kernel.org" <linux-fbdev@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Michael Kelley <mikelley@microsoft.com>,
        Wei Hu <weh@microsoft.com>
Subject: RE: [PATCH] video: hyperv_fb: Directly use the MMIO VRAM
Thread-Topic: [PATCH] video: hyperv_fb: Directly use the MMIO VRAM
Thread-Index: AQHWwBYrh+ep75sJdUq4vnjYcMIHRqnW9RcA
Date:   Tue, 24 Nov 2020 08:33:32 +0000
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References: <20201121014547.54890-1-decui@microsoft.com>
 <20201121145411.GG3025@boqun-archlinux>
In-Reply-To: <20201121145411.GG3025@boqun-archlinux>
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Hi Wei Liu,
Please do not pick up this patch, because actually MMIO VRAM can not work
with fb_deferred_io.

Previously I didn't test Xorg -- sorry. As soon as I tested it, I got the b=
elow
warning and the Xorg program ternimated immediately:

[   28.148432] WARNING: CPU: 19 PID: 1410 at mm/vmalloc.c:383 vmalloc_to_pa=
ge+0x14b/0x150
...
[   28.192959] CPU: 19 PID: 1410 Comm: Xorg Tainted: G            E     5.1=
0.0-rc1+ #4
...
[   28.208720] RIP: 0010:vmalloc_to_page+0x14b/0x150
...
[   28.299231] Call Trace:
[   28.301428]  fb_deferred_io_fault+0x3a/0xa0
[   28.305276]  __do_fault+0x36/0x120
[   28.308276]  handle_mm_fault+0x1144/0x1950
[   28.311963]  exc_page_fault+0x290/0x510
[   28.315551]  ? asm_exc_page_fault+0x8/0x30
[   28.319186]  asm_exc_page_fault+0x1e/0x30
[   28.322969] RIP: 0033:0x7fbeda3ec2f5

The issue is that fb_deferred_io_page() requires that the PFN be backed by =
a
struct page, but it looks the MMIO address does not have the struct page ba=
cked.

So I have to drop this patch.=20
Thanks Wei Hu and Michael for pointing this out!

FYI: drivers/video/fbdev/core/fb_defio.c:
static struct page *fb_deferred_io_page(struct fb_info *info, unsigned long=
 offs)
{
        void *screen_base =3D (void __force *) info->screen_base;
        struct page *page;

        if (is_vmalloc_addr(screen_base + offs))
                page =3D vmalloc_to_page(screen_base + offs);
        else
                page =3D pfn_to_page((info->fix.smem_start + offs) >> PAGE_=
SHIFT);

        return page;
}

/* this is to find and return the vmalloc-ed fb pages */
static vm_fault_t fb_deferred_io_fault(struct vm_fault *vmf)
{
        unsigned long offset;
        struct page *page;
        struct fb_info *info =3D vmf->vma->vm_private_data;

        offset =3D vmf->pgoff << PAGE_SHIFT;
        if (offset >=3D info->fix.smem_len)
                return VM_FAULT_SIGBUS;

        page =3D fb_deferred_io_page(info, offset);
        if (!page)
                return VM_FAULT_SIGBUS;

Thanks,
-- Dexuan