Re: [PATCH can] can: mcp251xfd: mcp251xfd_ring_alloc(): fix coalescing configuration when switching CAN modes

[Simon Horman <horms@kernel.org>]

On Fri, Oct 25, 2024 at 04:47:19PM +0200, Marc Kleine-Budde wrote:
> Since commit 50ea5449c563 ("can: mcp251xfd: fix ring configuration
> when switching from CAN-CC to CAN-FD mode"), the current ring and
> coalescing configuration is passed to can_ram_get_layout(). That fixed
> the issue when switching between CAN-CC and CAN-FD mode with
> configured ring (rx, tx) and/or coalescing parameters (rx-frames-irq,
> tx-frames-irq).
> 
> However 50ea5449c563 ("can: mcp251xfd: fix ring configuration when
> switching from CAN-CC to CAN-FD mode"), introduced a regression when
> switching CAN modes with disabled coalescing configuration: Even if
> the previous CAN mode has no coalescing configured, the new mode is
> configured with active coalescing. This leads to delayed receiving of
> CAN-FD frames.
> 
> This comes from the fact, that ethtool uses usecs = 0 and max_frames =
> 1 to disable coalescing, however the driver uses internally
> priv->{rx,tx}_obj_num_coalesce_irq = 0 to indicate disabled
> coalescing.
> 
> Fix the regression by assigning struct ethtool_coalesce
> ec->{rx,tx}_max_coalesced_frames_irq = 1 if coalescing is disabled in
> the driver as can_ram_get_layout() expects this.
> 
> Reported-by: https://github.com/vdh-robothania
> Closes: https://github.com/raspberrypi/linux/issues/6407
> Fixes: 50ea5449c563 ("can: mcp251xfd: fix ring configuration when switching from CAN-CC to CAN-FD mode")
> Cc: stable@vger.kernel.org
> Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

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