Re: [PATCH v2] ata: libata-core: Ignore forced transfer modes the host does not support

[Niklas Cassel]

On Thu, Oct 22, 2026 at 09:55:35AM +0200, Damien Le Moal wrote:
> On 2026/10/15 21:49, Rand Deeb wrote:
> > The libata.force parameter can force a transfer mode (pio0-6, mwdma0-4,
> > udma0-7) for a port or device. ata_force_xfermask() applies it in
> > ata_do_set_mode() right after ata_dev_xfermask() has limited the device
> > masks to what the host supports. It replaces those masks with the forced
> > mode without checking the host masks, so a mode the controller does not
> > support is passed to the driver's ->set_piomode() or ->set_dmamode().
> > 
> > Several PATA drivers rely on the host masks to keep the mode number in
> > range when indexing their timing tables. ata_piix, pata_efar,
> > pata_it8213, pata_oldpiix, pata_radisys and pata_rdc declare ATA_PIO4
> > and ATA_MWDMA12_ONLY, index a five-entry PIO timing table with
> > pio_mode - XFER_PIO_0, and index a three-entry needed_pio[] table with
> > dma_mode - XFER_MW_DMA_0. Booting with libata.force=pio5, pio6, mwdma3
> > or mwdma4 makes them read past those tables, use the values read as
> > further indices into the timing table, and write the result to the
> > controller's timing registers.
> > 
> > Make ata_force_xfermask() skip, with a warning, a forced transfer mode
> > that is not in the port's PIO, MWDMA or UDMA mask, and keep looking for
> > an earlier matching libata.force entry, as it already does for entries
> > that do not apply to the device. With libata.force=pio4,1.0:pio5 on a
> > PIO4 host, device 1.0 now gets pio4 instead of pio5. Forcing a mode the
> > host supports still works as before, including a mode that
> > ata_dev_xfermask() removed for the device or cable.
> > 
> > Found by Linux Verification Center (linuxtesting.org) with SVACE.
> > 
> > Fixes: 332673257056 ("libata: implement libata.force module parameter")
> > Signed-off-by: Rand Deeb <rand.sec96@xxxxxxxxx>
> 
> Looks good.
> 
> Reviewed-by: Damien Le Moal <dlemoal@xxxxxxxxxx>

Repushed commit to pick up Damien's tag.

Code unchanged.


Kind regards,
Niklas