Re: [Intel-wired-lan] [PATCH iwl-net v2] i40e: fix Jumbo Frame support after iPXE boot

[Simon Horman <horms@kernel.org>]

On Fri, Aug 15, 2025 at 05:20:39PM -0700, Jacob Keller wrote:
> The i40e hardware has multiple hardware settings which define the Maximum
> Frame Size (MFS) of the physical port. The firmware has an AdminQ command
> (0x0603) to configure the MFS, but the i40e Linux driver never issues this
> command.
> 
> In most cases this is no problem, as the NVM default value has the device
> configured for its maximum value of 9728. Unfortunately, recent versions of
> the iPXE intelxl driver now issue the 0x0603 Set Mac Config command,
> modifying the MFS and reducing it from its default value of 9728.
> 
> This occurred as part of iPXE commit 6871a7de705b ("[intelxl] Use admin
> queue to set port MAC address and maximum frame size"), a prerequisite
> change for supporting the E800 series hardware in iPXE. Both the E700 and
> E800 firmware support the AdminQ command, and the iPXE code shares much of
> the logic between the two device drivers.
> 
> The ice E800 Linux driver already issues the 0x0603 Set Mac Config command
> early during probe, and is thus unaffected by the iPXE change.
> 
> Since commit 3a2c6ced90e1 ("i40e: Add a check to see if MFS is set"), the
> i40e driver does check the I40E_PRTGL_SAH register, but it only logs a
> warning message if its value is below the 9728 default. This register also
> only covers received packets and not transmitted packets. A warning can
> inform system administrators, but does not correct the issue. No
> interactions from userspace cause the driver to write to PRTGL_SAH or issue
> the 0x0603 AdminQ command. Only a GLOBR reset will restore the value to its
> default value. There is no obvious method to trigger a GLOBR reset from
> user space.
> 
> To fix this, introduce the i40e_aq_set_mac_config() function, similar to
> the one from the ice driver. Call this during early probe to ensure that
> the device configuration matches driver expectation.
> 
> In addition, instead of just checking the I40E_PRTGL_SAH register, update
> its value to the 9728 default and write it back. This ensures that the
> hardware is in the expected state, regardless of whether the iPXE (or any
> other early boot driver) has modified this state.
> 
> This is a better user experience, as we now fix the issues with larger MTU
> instead of merely warning. It also aligns with the way the ice E800 series
> driver works.
> 
> A final note: The Fixes tag provided here is not strictly accurate. The
> issue occurs as a result of an external entity (the iPXE intelxl driver),
> and this is not a regression specifically caused by the mentioned change.
> However, I believe the original change to just warn about PRTGL_SAH being
> too low was an insufficient fix.
> 
> Fixes: 3a2c6ced90e1 ("i40e: Add a check to see if MFS is set")
> Link: https://github.com/ipxe/ipxe/commit/6871a7de705b6f6a4046f0d19da9bcd689c3bc8e
> Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>

Reviewed-by: Simon Horman <horms@kernel.org>