Re: [PATCH v2 1/4] ata: libata-core: Cache the general purpose log directory

[Niklas Cassel <cassel@kernel.org>]

On Thu, Jul 03, 2025 at 07:36:19PM +0900, Damien Le Moal wrote:
> The function ata_log_supported() tests if a log page is supported by a
> device using the General Purpose Log Directory log page, which lists the
> size of all surported log pages. However, this log page is read from the
> device using ata_read_log_page() every time ata_log_supported() is
> called. That is not necessary.
> 
> Avoid reading the General Purpose Log Directory log page by caching its
> content in the gp_log_dir buffer defined as part of struct ata_device.
> The functions ata_read_log_directory() and ata_clear_log_directory() are
> introduced to manage this buffer. ata_clear_log_directory() zero-fill
> the gp_log_dir buffer every time ata_dev_configure() is called, that is,
> when the device is first scanned and when it is being revalidated.
> The function ata_log_supported() is modified to call
> ata_read_log_directory() instead of ata_read_log_page().
> 
> The function ata_read_log_directory() calls ata_read_log_page() to read
> the General Purpose Log Directory log page from the device only if the
> first 16-bits word of the log is not equal to 0x0001, that is, it is not
> equal to the ACS mandated value for the log version.
> 
> With this, the log page is read from the device only once for every
> ata_dev_configure() call. For instance, with pr_debug enabled, a call
> to ata_dev_configure() before this patch generates the following log
> page accesses:
> 
> ata3.00: read log page - log 0x0, page 0x0
> ata3.00: read log page - log 0x13, page 0x0
> ata3.00: read log page - log 0x0, page 0x0
> ata3.00: read log page - log 0x12, page 0x0
> ata3.00: read log page - log 0x0, page 0x0
> ata3.00: read log page - log 0x30, page 0x0
> ata3.00: read log page - log 0x30, page 0x8
> ata3.00: read log page - log 0x0, page 0x0
> ata3.00: read log page - log 0x0, page 0x0
> ata3.00: read log page - log 0x0, page 0x0
> ata3.00: read log page - log 0x30, page 0x0
> ata3.00: read log page - log 0x0, page 0x0
> ata3.00: read log page - log 0x30, page 0x0
> ata3.00: read log page - log 0x30, page 0x3
> ata3.00: read log page - log 0x30, page 0x4
> ata3.00: read log page - log 0x18, page 0x0
> 
> That is, the general purpose log directory page is read 7 times.
> With this patch applied, the number of accesses to this log page is
> reduced to one:
> 
> ata3.00: read log page - log 0x0, page 0x0
> ata3.00: read log page - log 0x13, page 0x0
> ata3.00: read log page - log 0x12, page 0x0
> ata3.00: read log page - log 0x30, page 0x0
> ata3.00: read log page - log 0x30, page 0x8
> ata3.00: read log page - log 0x30, page 0x0
> ata3.00: read log page - log 0x30, page 0x0
> ata3.00: read log page - log 0x30, page 0x3
> ata3.00: read log page - log 0x30, page 0x4
> ata3.00: read log page - log 0x18, page 0x0
> 
> Signed-off-by: Damien Le Moal <dlemoal@kernel.org>

Reviewed-by: Niklas Cassel <cassel@kernel.org>