Re: [PATCH v2 3/6] tpm_tis_spi: add max xfer size

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On 17.07.2019 00:45, Stephen Boyd wrote:
From: Andrey Pronin <apronin@xxxxxxxxxxxx>

Reject burstcounts larger than 64 bytes reported by tpm.

This is not the correct thing to do here. To quote the specification:

"burstCount is defined as the number of bytes that can be written to or read from the data FIFO by the software without incurring a wait state." (https://trustedcomputinggroup.org/wp-content/uploads/TCG_PC_Client_Platform_TPM_Profile_PTP_2.0_r1.03_v22.pdf Page 84)

If the FIFO contains 1k of data, it is completely valid for the TPM to report that as its burstCount, there is no need to arbitrarily limit it.

Also, burstCount is a property of the high-level TIS protocol, that should not really care whether the low-level transfers are done via LPC or SPI (or I2C). Since tpm_tis_spi can only transfer 64 bytes at a time, it is its job to split larger transfers (which it does perfectly fine). This also has the advantage that burstCount needs only to be read once, and then we can do 16 SPI transfers in a row to read that 1k of data. With your change, it will read 64 bytes, then read burstCount again, before reading the next 64 bytes and so on. This unnecessarily limits performance.

Maybe you can describe the problem you're trying to solve in more detail, so that a better solution can be found, since this is clearly something not intended by the spec.

Alexander



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