linux-kernel - Re: [PATCH] drm/hyperv: Don't rely on screen_info.lfb

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Date:   Mon, 12 Sep 2022 09:03:53 +0200
From:   Thomas Zimmermann <tzimmermann@...e.de>
To:     Saurabh Singh Sengar <ssengar@...ux.microsoft.com>
Cc:     linux-hyperv@...r.kernel.org, airlied@...ux.ie,
        ssengar@...rosoft.com, linux-kernel@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, mikelley@...rosoft.com,
        drawat.floss@...il.com
Subject: Re: [PATCH] drm/hyperv: Don't rely on screen_info.lfb_base for Gen1
 VMs

Hi

Am 11.09.22 um 18:21 schrieb Saurabh Singh Sengar:
> On Sat, Sep 10, 2022 at 08:11:24PM +0200, Thomas Zimmermann wrote:
>> Hi
>>
>> Am 09.09.22 um 16:43 schrieb Saurabh Sengar:
>>> hyperv_setup_vram tries to remove conflicting framebuffer based on
>>> 'screen_info'. As observed in past due to some bug or wrong setting
>>> in grub, the 'screen_info' fields may not be set for Gen1, and in such
>>> cases drm_aperture_remove_conflicting_framebuffers will not do anything
>>> useful.
>>> For Gen1 VMs, it should always be possible to get framebuffer
>>> conflict removed using PCI device instead.
>>>
>>> Fixes: a0ab5abced55 ("drm/hyperv : Removing the restruction of VRAM allocation with PCI bar size")
>>> Signed-off-by: Saurabh Sengar <ssengar@...ux.microsoft.com>
>>> ---
>>>   drivers/gpu/drm/hyperv/hyperv_drm_drv.c | 24 ++++++++++++++++++++----
>>>   1 file changed, 20 insertions(+), 4 deletions(-)
>>>
>>> diff --git a/drivers/gpu/drm/hyperv/hyperv_drm_drv.c b/drivers/gpu/drm/hyperv/hyperv_drm_drv.c
>>> index 6d11e7938c83..b0cc974efa45 100644
>>> --- a/drivers/gpu/drm/hyperv/hyperv_drm_drv.c
>>> +++ b/drivers/gpu/drm/hyperv/hyperv_drm_drv.c
>>> @@ -73,12 +73,28 @@ static int hyperv_setup_vram(struct hyperv_drm_device *hv,
>>>   			     struct hv_device *hdev)
>>>   {
>>>   	struct drm_device *dev = &hv->dev;
>>> +	struct pci_dev *pdev;
>>>   	int ret;
>>> -	drm_aperture_remove_conflicting_framebuffers(screen_info.lfb_base,
>>> -						     screen_info.lfb_size,
>>> -						     false,
>>> -						     &hyperv_driver);
>>> +	if (efi_enabled(EFI_BOOT)) {
>>> +		drm_aperture_remove_conflicting_framebuffers(screen_info.lfb_base,
>>> +							     screen_info.lfb_size,
>>
>> Using screen_info here seems wrong in any case. You want to remove
>> the framebuffer devices that conflict with your driver, which might
>> be unrelated to screen_info. AFAICT the correct solution would
>> always retrieve the PCI device for removal (i.e., always do the else
>> branch).
> 
> In a Gen2 VM, the Hyper-V frame buffer device is presented only as a VMbus device.
> It's not presented as a PCI device like it is in a Gen1 VM. This would have worked
> if we had the frame buffer device available as PCI device in Gen2 but unfortunately
> thats not the case here.

Thanks for explaining. There is an instance of struct hv_device passed 
to the probe function. I suspect you cannot get the framebuffer range 
from this instance (e.g., via the device's platform_data)?

If you absolutely can't get the actual memory region from the device, 
it's better to remove all framebuffers via 
drm_aperture_remove_framebuffers() than to use screen_info.

Best regards
Thomas

> 
> Regards,
> Saurabh
> 
>>
>> Best regard
>> Thomas
>>
>>> +							     false,
>>> +							     &hyperv_driver);
>>> +	} else {
>>> +		pdev = pci_get_device(PCI_VENDOR_ID_MICROSOFT, PCI_DEVICE_ID_HYPERV_VIDEO, NULL);
>>> +		if (!pdev) {
>>> +			drm_err(dev, "Unable to find PCI Hyper-V video\n");
>>> +			return -ENODEV;
>>> +		}
>>> +
>>> +		ret = drm_aperture_remove_conflicting_pci_framebuffers(pdev, &hyperv_driver);
>>> +		pci_dev_put(pdev);
>>> +		if (ret) {
>>> +			drm_err(dev, "Not able to remove boot fb\n");
>>> +			return ret;
>>> +		}
>>> +	}
>>>   	hv->fb_size = (unsigned long)hv->mmio_megabytes * 1024 * 1024;
>>
>> -- 
>> Thomas Zimmermann
>> Graphics Driver Developer
>> SUSE Software Solutions Germany GmbH
>> Maxfeldstr. 5, 90409 Nürnberg, Germany
>> (HRB 36809, AG Nürnberg)
>> Geschäftsführer: Ivo Totev
> 
> 
> 

-- 
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Ivo Totev

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