Re: [RFC PATCH v1 34/57] sata_sil24: Remove PAGE_SIZE compile-time constant assumption

Ryan Roberts
2024-10-17T12:59:40Z

On 17/10/2024 10:09, Niklas Cassel wrote:
> Hello Ryan,
>
> While I realize that this has not always been consistent,
> please prefix the subject with "ata: ", so that it becomes
> "ata: sata_sil24: ".

Noted; I'll fix this in the next version.

>
> On Mon, Oct 14, 2024 at 11:58:41AM +0100, Ryan Roberts wrote:
>> To prepare for supporting boot-time page size selection, refactor code
>> to remove assumptions about PAGE_SIZE being compile-time constant. Code
>> intended to be equivalent when compile-time page size is active.
>>
>> Convert "struct sil24_ata_block" and "struct sil24_atapi_block" to use a
>> flexible array member for their sge[] array. The previous static size of
>> SIL24_MAX_SGE depends on PAGE_SIZE so doesn't work for boot-time page
>> size.
>>
>> Wrap global variables that are initialized with PAGE_SIZE derived values
>> using DEFINE_GLOBAL_PAGE_SIZE_VAR() so their initialization can be
>> deferred for boot-time page size builds.
>>
>> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
>> ---
>>
>> ***NOTE***
>> Any confused maintainers may want to read the cover note here for context:
>> https://lore.kernel.org/all/20241014105514.3206191-1-ryan.roberts@arm.com/
>>
>> drivers/ata/sata_sil24.c | 46 +++++++++++++++++++---------------------
>> 1 file changed, 22 insertions(+), 24 deletions(-)
>>
>> diff --git a/drivers/ata/sata_sil24.c b/drivers/ata/sata_sil24.c
>> index 72c03cbdaff43..85c6382976626 100644
>> --- a/drivers/ata/sata_sil24.c
>> +++ b/drivers/ata/sata_sil24.c
>> @@ -42,26 +42,25 @@ struct sil24_sge {
>> __le32 flags;
>> };
>>
>> +/*
>> + * sil24 fetches in chunks of 64bytes. The first block
>> + * contains the PRB and two SGEs. From the second block, it's
>> + * consisted of four SGEs and called SGT. Calculate the
>> + * number of SGTs that fit into one page.
>> + */
>> +#define SIL24_PRB_SZ (sizeof(struct sil24_prb) + 2 * sizeof(struct sil24_sge))
>> +#define SIL24_MAX_SGT ((PAGE_SIZE - SIL24_PRB_SZ) / (4 * sizeof(struct sil24_sge)))
>> +
>> +/*
>> + * This will give us one unused SGEs for ATA. This extra SGE
>> + * will be used to store CDB for ATAPI devices.
>> + */
>> +#define SIL24_MAX_SGE (4 * SIL24_MAX_SGT + 1)
>>
>> enum {
>> SIL24_HOST_BAR = 0,
>> SIL24_PORT_BAR = 2,
>>
>> - /* sil24 fetches in chunks of 64bytes. The first block
>> - * contains the PRB and two SGEs. From the second block, it's
>> - * consisted of four SGEs and called SGT. Calculate the
>> - * number of SGTs that fit into one page.
>> - */
>> - SIL24_PRB_SZ = sizeof(struct sil24_prb)
>> - + 2 * sizeof(struct sil24_sge),
>> - SIL24_MAX_SGT = (PAGE_SIZE - SIL24_PRB_SZ)
>> - / (4 * sizeof(struct sil24_sge)),
>> -
>> - /* This will give us one unused SGEs for ATA. This extra SGE
>> - * will be used to store CDB for ATAPI devices.
>> - */
>> - SIL24_MAX_SGE = 4 * SIL24_MAX_SGT + 1,
>> -
>> /*
>> * Global controller registers (128 bytes @ BAR0)
>> */
>> @@ -244,13 +243,13 @@ enum {
>>
>> struct sil24_ata_block {
>> struct sil24_prb prb;
>> - struct sil24_sge sge[SIL24_MAX_SGE];
>> + struct sil24_sge sge[];
>> };
>>
>> struct sil24_atapi_block {
>> struct sil24_prb prb;
>> u8 cdb[16];
>> - struct sil24_sge sge[SIL24_MAX_SGE];
>> + struct sil24_sge sge[];
>> };
>>
>> union sil24_cmd_block {
>> @@ -373,7 +372,7 @@ static struct pci_driver sil24_pci_driver = {
>> #endif
>> };
>>
>> -static const struct scsi_host_template sil24_sht = {
>> +static DEFINE_GLOBAL_PAGE_SIZE_VAR_CONST(struct scsi_host_template, sil24_sht, {
>> __ATA_BASE_SHT(DRV_NAME),
>> .can_queue = SIL24_MAX_CMDS,
>> .sg_tablesize = SIL24_MAX_SGE,
>> @@ -382,7 +381,7 @@ static const struct scsi_host_template sil24_sht = {
>> .sdev_groups = ata_ncq_sdev_groups,
>> .change_queue_depth = ata_scsi_change_queue_depth,
>> .device_configure = ata_scsi_device_configure
>> -};
>> +});
>>
>> static struct ata_port_operations sil24_ops = {
>> .inherits = &sata_pmp_port_ops,
>> @@ -1193,7 +1192,7 @@ static int sil24_port_start(struct ata_port *ap)
>> struct device *dev = ap->host->dev;
>> struct sil24_port_priv *pp;
>> union sil24_cmd_block *cb;
>> - size_t cb_size = sizeof(*cb) * SIL24_MAX_CMDS;
>> + size_t cb_size = PAGE_SIZE * SIL24_MAX_CMDS;
>> dma_addr_t cb_dma;
>>
>> pp = devm_kzalloc(dev, sizeof(*pp), GFP_KERNEL);
>> @@ -1258,7 +1257,6 @@ static void sil24_init_controller(struct ata_host *host)
>>
>> static int sil24_init_one(struct pci_dev *pdev, const struct pci_device_id *ent)
>> {
>> - extern int __MARKER__sil24_cmd_block_is_sized_wrongly;
>> struct ata_port_info pi = sil24_port_info[ent->driver_data];
>> const struct ata_port_info *ppi[] = { &pi, NULL };
>> void __iomem * const *iomap;
>> @@ -1266,9 +1264,9 @@ static int sil24_init_one(struct pci_dev *pdev, const struct pci_device_id *ent)
>> int rc;
>> u32 tmp;
>>
>> - /* cause link error if sil24_cmd_block is sized wrongly */
>> - if (sizeof(union sil24_cmd_block) != PAGE_SIZE)
>> - __MARKER__sil24_cmd_block_is_sized_wrongly = 1;
>> + /* union sil24_cmd_block must be PAGE_SIZE */
>> + BUG_ON(struct_size_t(struct sil24_atapi_block, sge, SIL24_MAX_SGE) != PAGE_SIZE);
>> + BUG_ON(struct_size_t(struct sil24_ata_block, sge, SIL24_MAX_SGE) > PAGE_SIZE);
>>
>> ata_print_version_once(&pdev->dev, DRV_VERSION);
>>
>> --
>> 2.43.0
>>
>
> As you might know, there is an effort to annotate all flexible array
> members with their run-time size information, see commit:
> dd06e72e68bc ("Compiler Attributes: Add __counted_by macro")

I'm vaguely aware of it. But as I understand it, __counted_by() nominates
another member in the struct which keeps the count? In this case, there is no
such member, it's size is implicit based on the value of PAGE_SIZE. So I'm not
sure if it's practical to use it here?

>
> I haven't looked at the DEFINE_GLOBAL_PAGE_SIZE_VAR_CONST macro, but since

DEFINE_GLOBAL_PAGE_SIZE_VAR_CONST(), when doing a boot-time page size build,
defers the initialization of the global variable to kernel init time, when
PAGE_SIZE is known. Because SIL24_MAX_SGE is defined in terms of PAGE_SIZE, this
deferral is required.

> sge[] now becomes a flexible array member, I think it would be nice if it
> would be possible to somehow use the __counted_by macro.
>
> Other than that, this looks good to me.

Thanks for the review!

>
>
> Kind regards,
> Niklas