Re: [PATCH v2 2/2] net: macb: Limit maximum GEM TX length in TSO

[David Miller]

From: Harini Katakam <harini.katakam@xxxxxxxxxx>
Date: Mon,  3 Feb 2020 18:48:02 +0530

> GEM_MAX_TX_LEN currently resolves to 0x3FF8 for any IP version supporting
> TSO with full 14bits of length field in payload descriptor. But an IP
> errata causes false amba_error (bit 6 of ISR) when length in payload
> descriptors is specified above 16387. The error occurs because the DMA
> falsely concludes that there is not enough space in SRAM for incoming
> payload. These errors were observed continuously under stress of large
> packets using iperf on a version where SRAM was 16K for each queue. This
> errata will be documented shortly and affects all versions since TSO
> functionality was added. Hence limit the max length to 0x3FC0 (rounded).
> 
> Signed-off-by: Harini Katakam <harini.katakam@xxxxxxxxxx>
> ---
> v2:
> Use 0x3FC0 by default

You should add a comment above the definition which explains how this
value was derived.  It looks magic currently.