I have a latency issue using a SPI-based TPM chip with tpm_tis driver from non-rt usermode application, which induces ~400 us latency spikes in cyclictest (Intel Atom E3940 system, PREEMPT_RT_FULL kernel). The spikes are caused by a stalling ioread8() operation, following a sequence of 30+ iowrite8()s to the same address. I believe this happens because the writes are cached (in cpu or somewhere along the bus), which gets flushed on the first LOAD instruction (ioread*()) that follows. The enclosed change appears to fix this issue: read the TPM chip's access register (status code) after every iowrite*() operation. I believe this works because it amortize the cost of flushing data to chip across multiple instructions. However, I don't have any direct evidence to support this theory. Does this seem like a reasonable theory? Any feedback on the change (a better way to do it, perhaps)? Thanks, Haris Okanovic https://github.com/harisokanovic/linux/tree/dev/hokanovi/tpm-latency-spike-fix-rfc --- drivers/char/tpm/tpm_tis.c | 18 +++++++++++++++++- 1 file changed, 17 insertions(+), 1 deletion(-) diff --git a/drivers/char/tpm/tpm_tis.c b/drivers/char/tpm/tpm_tis.c index c7e1384f1b08..5cdbfec0ad67 100644 --- a/drivers/char/tpm/tpm_tis.c +++ b/drivers/char/tpm/tpm_tis.c @@ -89,6 +89,19 @@ static inline int is_itpm(struct acpi_device *dev) } #endif +#ifdef CONFIG_PREEMPT_RT_FULL +/* + * Flushes previous iowrite*() operations to chip so that a subsequent + * ioread*() won't stall a cpu. + */ +static void tpm_tcg_flush(struct tpm_tis_tcg_phy *phy) +{ + ioread8(phy->iobase + TPM_ACCESS(0)); +} +#else +#define tpm_tcg_flush do { } while(0) +#endif + static int tpm_tcg_read_bytes(struct tpm_tis_data *data, u32 addr, u16 len, u8 *result) { @@ -104,8 +117,10 @@ static int tpm_tcg_write_bytes(struct tpm_tis_data *data, u32 addr, u16 len, { struct tpm_tis_tcg_phy *phy = to_tpm_tis_tcg_phy(data); - while (len--) + while (len--) { iowrite8(*value++, phy->iobase + addr); + tpm_tcg_flush(phy); + } return 0; } @@ -130,6 +145,7 @@ static int tpm_tcg_write32(struct tpm_tis_data *data, u32 addr, u32 value) struct tpm_tis_tcg_phy *phy = to_tpm_tis_tcg_phy(data); iowrite32(value, phy->iobase + addr); + tpm_tcg_flush(phy); return 0; } -- 2.13.2 -- To unsubscribe from this list: send the line "unsubscribe linux-rt-users" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
