Re: [PATCH] drm/xe/pf: Keep VF LMEM BAR size low if no VFs enabled

[Michal Wajdeczko <michal.wajdeczko@intel.com>]

On 9/18/2025 6:43 PM, Marcin Bernatowicz wrote:
> When VFs are enabled on dGFX the driver resizes the PF VF_LMEM_BAR to
> fit the requested layout. After VFs are disabled the PF VF BAR
> size is left as-is. On platforms with tight MMIO apertures a
> subsequent unplug/rescan followed by another enable may fail with:
> 
>   "VF BAR …: can't assign; no space"
> 
> because the PCI core reserves address space based on the (now large) VF
> template, often multiplied by totalvfs.
> 
> v2: Use pci.total_vfs in helper (Michał Wajdeczko)

internal reviews don't count

> 
> Closes: https://gitlab.freedesktop.org/drm/xe/kernel/-/issues/5937
> Fixes: 94eae6ee4c2d ("drm/xe/pf: Set VF LMEM BAR size")
> Signed-off-by: Marcin Bernatowicz <marcin.bernatowicz@linux.intel.com>
> Cc: Michał Wajdeczko <michal.wajdeczko@intel.com>
> Cc: Michał Winiarski <michal.winiarski@intel.com>
> ---
>  drivers/gpu/drm/xe/xe_pci_sriov.c | 22 ++++++++++++++++++++++
>  1 file changed, 22 insertions(+)
> 
> diff --git a/drivers/gpu/drm/xe/xe_pci_sriov.c b/drivers/gpu/drm/xe/xe_pci_sriov.c
> index af05db07162e..ff003a650f79 100644
> --- a/drivers/gpu/drm/xe/xe_pci_sriov.c
> +++ b/drivers/gpu/drm/xe/xe_pci_sriov.c
> @@ -144,11 +144,26 @@ static int resize_vf_vram_bar(struct xe_device *xe, int num_vfs)
>  	return pci_iov_vf_bar_set_size(pdev, VF_LMEM_BAR, __fls(sizes));
>  }
>  
> +static void reduce_vf_vram_bar_size(struct xe_device *xe)

is it "reduce" or "restore" ?

> +{
> +	struct pci_dev *pdev = to_pci_dev(xe->drm.dev);
> +	int err;
> +
> +	if (!IS_DGFX(xe))
> +		return;
> +
> +	err = resize_vf_vram_bar(xe, pci_sriov_get_totalvfs(pdev));

pci_sriov_get_totalvfs() might also return tweaked value based on xe.max_vfs modparam

https://elixir.bootlin.com/linux/v6.17-rc6/source/drivers/pci/iov.c#L1276

> +	if (err)
> +		xe_sriov_info(xe, "Failed to reduce VF LMEM BAR size: %d\n",
> +			      err);

we try to show nice error codes using (%pe)

> +}
> +
>  static int pf_enable_vfs(struct xe_device *xe, int num_vfs)
>  {
>  	struct pci_dev *pdev = to_pci_dev(xe->drm.dev);
>  	int total_vfs = xe_sriov_pf_get_totalvfs(xe);
>  	int err;
> +	bool vf_vram_bar_resized = false;

please try to keep vars in reverse-xmas-tree order

>  
>  	xe_assert(xe, IS_SRIOV_PF(xe));
>  	xe_assert(xe, num_vfs > 0);
> @@ -178,6 +193,8 @@ static int pf_enable_vfs(struct xe_device *xe, int num_vfs)
>  		err = resize_vf_vram_bar(xe, num_vfs);
>  		if (err)
>  			xe_sriov_info(xe, "Failed to set VF LMEM BAR size: %d\n", err);
> +		else
> +			vf_vram_bar_resized = true;
>  	}
>  
>  	err = pci_enable_sriov(pdev, num_vfs);
> @@ -194,6 +211,9 @@ static int pf_enable_vfs(struct xe_device *xe, int num_vfs)
>  	return num_vfs;
>  
>  failed:
> +	if (vf_vram_bar_resized)
> +		reduce_vf_vram_bar_size(xe);

in pf_disable_vfs() below it's called unconditionally
why can't we do the same here? or it's broken there?

> +
>  	pf_unprovision_vfs(xe, num_vfs);
>  	xe_pm_runtime_put(xe);
>  out:
> @@ -218,6 +238,8 @@ static int pf_disable_vfs(struct xe_device *xe)
>  
>  	pci_disable_sriov(pdev);
>  
> +	reduce_vf_vram_bar_size(xe);
> +
>  	pf_reset_vfs(xe, num_vfs);
>  
>  	pf_unprovision_vfs(xe, num_vfs);