Date:   Wed, 4 Oct 2023 20:55:04 +0800
From:   Sui Jingfeng <suijingfeng@...ngson.cn>
To:     Bjorn Helgaas <helgaas@...nel.org>
Cc:     Bjorn Helgaas <bhelgaas@...gle.com>, linux-pci@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH pci-next v6 1/2] PCI/VGA: Make the
 vga_is_firmware_default() less arch-dependent

Hi,


On 2023/10/3 23:54, Bjorn Helgaas wrote:
> On Mon, Oct 02, 2023 at 08:05:10PM +0800, Sui Jingfeng wrote:
>> Currently, the vga_is_firmware_default() function only works on x86 and
>> ia64, it is a no-op on the rest of the architectures. This patch completes
>> the implementation for it, the added code tries to capture the PCI (e) VGA
>> device that owns the firmware framebuffer, since only one GPU could own
>> the firmware fb, things are almost done once we have determined the boot
>> VGA device. As the PCI resource relocation do have a influence on the
>> results of identification, we make it available on architectures where PCI
>> resource relocation does happen at first. Because this patch is more
>> important for those architectures(such as arm, arm64, loongarch, mips and
>> risc-v etc).
> There's a little too much going on this this patch.  The problem is
> very simple: currently we compare firmware BAR assignments with BARs
> that may have been reassigned by Linux.
>
> What if we did something like the patch below?  I think it will be
> less confusing if we only have one copy of the code related to
> screen_info.
>
> Bjorn
>
>> Signed-off-by: Sui Jingfeng <suijingfeng@...ngson.cn>
>> ---
>>   drivers/pci/vgaarb.c | 76 ++++++++++++++++++++++++++++++++++++++++++++
>>   1 file changed, 76 insertions(+)
>>
>> diff --git a/drivers/pci/vgaarb.c b/drivers/pci/vgaarb.c
>> index 5e6b1eb54c64..02821c0f4cd0 100644
>> --- a/drivers/pci/vgaarb.c
>> +++ b/drivers/pci/vgaarb.c
>> @@ -60,6 +60,9 @@ static bool vga_arbiter_used;
>>   static DEFINE_SPINLOCK(vga_lock);
>>   static DECLARE_WAIT_QUEUE_HEAD(vga_wait_queue);
>>   
>> +/* The PCI(e) VGA device which owns the firmware framebuffer */
>> +static struct pci_dev *pdev_boot_vga;
>> +
>>   static const char *vga_iostate_to_str(unsigned int iostate)
>>   {
>>   	/* Ignore VGA_RSRC_IO and VGA_RSRC_MEM */
>> @@ -582,6 +585,9 @@ static bool vga_is_firmware_default(struct pci_dev *pdev)
>>   
>>   		return true;
>>   	}
>> +#else
>> +	if (pdev_boot_vga && pdev_boot_vga == pdev)
>> +		return true;
>>   #endif
>>   	return false;
>>   }
>> @@ -1557,3 +1563,73 @@ static int __init vga_arb_device_init(void)
>>   	return rc;
>>   }
>>   subsys_initcall_sync(vga_arb_device_init);
>> +
>> +/*
>> + * Get the physical address range that the firmware framebuffer occupies.
>> + *
>> + * Note that the global screen_info is arch-specific, thus CONFIG_SYSFB is
>> + * chosen as compile-time conditional to suppress linkage problems on non-x86
>> + * architectures.
>> + *
>> + * Returns true on success, otherwise return false.
>> + */
>> +static bool vga_arb_get_firmware_fb_range(u64 *start, u64 *end)
>> +{
>> +	u64 fb_start = 0;
>> +	u64 fb_size = 0;
>> +	u64 fb_end;
>> +
>> +#if defined(CONFIG_X86) || defined(CONFIG_IA64) || defined(CONFIG_SYSFB)
>> +	fb_start = screen_info.lfb_base;
>> +	if (screen_info.capabilities & VIDEO_CAPABILITY_64BIT_BASE)
>> +		fb_start |= (u64)screen_info.ext_lfb_base << 32;
>> +
>> +	fb_size = screen_info.lfb_size;
>> +#endif
>> +
>> +	/* No firmware framebuffer support */
>> +	if (!fb_start || !fb_size)
>> +		return false;
>> +
>> +	fb_end = fb_start + fb_size - 1;
>> +
>> +	*start = fb_start;
>> +	*end = fb_end;
>> +
>> +	return true;
>> +}
>> +
>> +/*
>> + * Identify the PCI VGA device that contains the firmware framebuffer
>> + */
>> +static void pci_boot_vga_capturer(struct pci_dev *pdev)
>> +{
>> +	u64 fb_start, fb_end;
>> +	struct resource *res;
>> +	unsigned int i;
>> +
>> +	if (pdev_boot_vga)
>> +		return;
>> +
>> +	if (!vga_arb_get_firmware_fb_range(&fb_start, &fb_end))
>> +		return;
>> +
>> +	pci_dev_for_each_resource(pdev, res, i) {
>> +		if (resource_type(res) != IORESOURCE_MEM)
>> +			continue;
>> +
>> +		if (!res->start || !res->end)
>> +			continue;
>> +
>> +		if (res->start <= fb_start && fb_end <= res->end) {
>> +			pdev_boot_vga = pdev;
>> +
>> +			vgaarb_info(&pdev->dev,
>> +				    "BAR %u: %pR contains firmware FB [0x%llx-0x%llx]\n",
>> +				    i, res, fb_start, fb_end);
>> +			break;
>> +		}
>> +	}
>> +}
>> +DECLARE_PCI_FIXUP_CLASS_HEADER(PCI_ANY_ID, PCI_ANY_ID, PCI_CLASS_DISPLAY_VGA,
>> +			       8, pci_boot_vga_capturer);
>
> PCI/VGA: Match firmware framebuffer before BAR reassignment
>
> vga_is_firmware_default() decides a device is the firmware default VGA
> device if it has a BAR that contains the framebuffer described by
> screen_info.
>
> Previously this was unreliable because the screen_info framebuffer address
> comes from firmware, and the Linux PCI core may reassign device BARs before
> vga_is_firmware_default() runs.  This reassignment means the BAR may not
> match the screen_info values, but we still want to select the device as the
> firmware default.
>
> Make vga_is_firmware_default() more reliable by running it as a quirk so it
> happens before any BAR reassignment.
>
> diff --git a/drivers/pci/vgaarb.c b/drivers/pci/vgaarb.c
> index 5e6b1eb54c64..4a53e76caddd 100644
> --- a/drivers/pci/vgaarb.c
> +++ b/drivers/pci/vgaarb.c
> @@ -60,6 +60,8 @@ static bool vga_arbiter_used;
>   static DEFINE_SPINLOCK(vga_lock);
>   static DECLARE_WAIT_QUEUE_HEAD(vga_wait_queue);
>   
> +static struct pci_dev *vga_firmware_device;
> +
>   static const char *vga_iostate_to_str(unsigned int iostate)
>   {
>   	/* Ignore VGA_RSRC_IO and VGA_RSRC_MEM */
> @@ -560,6 +562,7 @@ static bool vga_is_firmware_default(struct pci_dev *pdev)
>   	u64 base = screen_info.lfb_base;
>   	u64 size = screen_info.lfb_size;
>   	struct resource *r;
> +	unsigned int i;
>   	u64 limit;
>   
>   	/* Select the device owning the boot framebuffer if there is one */
> @@ -570,7 +573,7 @@ static bool vga_is_firmware_default(struct pci_dev *pdev)
>   	limit = base + size;
>   
>   	/* Does firmware framebuffer belong to us? */
> -	pci_dev_for_each_resource(pdev, r) {
> +	pci_dev_for_each_resource(pdev, r, i) {
>   		if (resource_type(r) != IORESOURCE_MEM)
>   			continue;
>   
> @@ -580,6 +583,8 @@ static bool vga_is_firmware_default(struct pci_dev *pdev)
>   		if (base < r->start || limit >= r->end)
>   			continue;
>   
> +		vgaarb_info(&pdev->dev, "BAR %u: %pR contains firmware framebuffer [%#010llx-%#010llx]\n",
> +			    i, r, base, limit - 1);
>   		return true;
>   	}
>   #endif
> @@ -623,7 +628,7 @@ static bool vga_is_boot_device(struct vga_device *vgadev)
>   	if (boot_vga && boot_vga->is_firmware_default)
>   		return false;
>   
> -	if (vga_is_firmware_default(pdev)) {
> +	if (pdev == vga_firmware_device) {
>   		vgadev->is_firmware_default = true;
>   		return true;
>   	}
> @@ -1557,3 +1562,14 @@ static int __init vga_arb_device_init(void)
>   	return rc;
>   }
>   subsys_initcall_sync(vga_arb_device_init);
> +
> +static void vga_match_firmware_framebuffer(struct pci_dev *pdev)
> +{
> +	if (vga_firmware_device)
> +		return;
> +
> +	if (vga_is_firmware_default(pdev))
> +		vga_firmware_device = pdev;
> +}
> +DECLARE_PCI_FIXUP_CLASS_HEADER(PCI_ANY_ID, PCI_ANY_ID, PCI_CLASS_DISPLAY_VGA,
> +			       8, vga_match_firmware_framebuffer);


Q: What if we did something like the patch below?

A:

But the vga_is_firmware_default() function only works on X86 and IA64,
you patch doesn't solve the problems on ARM64 and LoongArch.

Q:

I think it will be
less confusing if we only have one copy of the code related to
screen_info.

A:

Since you have already know everything, you can do anything with those two patch
if you would like to pick them up. As I have spend enough more time on this, if
two copy of the screen_info really matters, Would you mind to squash those two
patch into one?

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