Re: [PATCH] ahci: document and clarify unconventional intel quirk

[Mika Westerberg <mika.westerberg@linux.intel.com>]

On Wed, Feb 07, 2024 at 10:10:53AM +0100, Niklas Cassel wrote:
> The ahci_intel_pcs_quirk is unconventional in several ways:
> First of all because it has a board ID for which the quirk should NOT be
> applied (board_ahci_pcs7), instead of the usual way where we have a board
> ID for which the quirk should be applied.
> 
> The second reason is that other than only excluding board_ahci_pcs7 from
> the quirk, PCI devices that make use of the generic entry in ahci_pci_tbl
> (which matches on AHCI class code) are also excluded.
> 
> This can of course lead to very subtle breakage, and did indeed do so in:
> commit 104ff59af73a ("ata: ahci: Add Tiger Lake UP{3,4} AHCI controller"),
> which added an explicit entry with board_ahci_low_power to ahci_pci_tbl.
> 
> This caused many users to complain that their SATA drives disappeared.
> The logical assumption was of course that the issue was related to LPM,
> and was therefore reverted in commit 6210038aeaf4 ("ata: ahci: Revert
> "ata: ahci: Add Tiger Lake UP{3,4} AHCI controller"").
> 
> It took a lot of time to figure out that this was all completely unrelated
> to LPM, and was instead caused by an unconventional Intel quirk.
> 
> While this quirk should definitely be cleaned up to be implemented like
> all other quirks, for now, at least document the behavior of this quirk.
> 
> Link: https://bugzilla.kernel.org/show_bug.cgi?id=217114
> Signed-off-by: Niklas Cassel <cassel@kernel.org>

Thanks!

Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com>