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

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Date:   Tue, 04 Feb 2020 10:37:49 +0100 (CET)
From:   David Miller <davem@...emloft.net>
To:     harini.katakam@...inx.com
Cc:     nicolas.ferre@...rochip.com, claudiu.beznea@...rochip.com,
        kuba@...nel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, michal.simek@...inx.com,
        harinikatakamlinux@...il.com
Subject: Re: [PATCH v2 2/2] net: macb: Limit maximum GEM TX length in TSO

From: Harini Katakam <harini.katakam@...inx.com>
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@...inx.com>
> ---
> v2:
> Use 0x3FC0 by default

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