[{"id":3676308,"web_url":"http://patchwork.ozlabs.org/comment/3676308/","msgid":"","list_archive_url":null,"date":"2026-04-12T15:35:57","subject":"Re: [PATCH] hw/net/fsl_etsec: implement RCTRL[CFA] FCS stripping on\n receive","submitter":{"id":93121,"url":"http://patchwork.ozlabs.org/api/people/93121/","name":"Vivien LEGER","email":"vivien.leger@gmail.com"},"content":"Please disregard the previous patch, it was incorrect.\nAfter further investigation, QEMU's fsl_etsec already includes the 4-byte\nFCS in bd->length, which is the correct hardware behaviour.\nThe Linux gianfar driver confirms this by unconditionally subtracting\nETH_FCS_LEN from bd->length on receive.\n Additionally, The RCTRL register has no FCS strip control bit. The actual\nissue was in my RTEMS lwIP driver which was not subtracting the FCS from\nbd->length before passing frames to the stack, causing ARP replies to fail\nlength validation.\nThat is a driver bug, not a QEMU bug. Sorry\n\nOn Sun, Apr 12, 2026 at 3:46 PM Vivien LEGER wrote:\n\n> Please disregard the previous patch.\n> The RCTRL[CFA] bit reference was incorrect; CFA appears in the DFVLAN\n> register context, not RCTRL.\n> The patch needs further research before resubmission.\n> Sorry for the noise.\n>\n> On Sun, Apr 12, 2026 at 3:40 PM Vivien LEGER \n> wrote:\n>\n>> The eTSEC hardware strips the 4-byte FCS from received frames by\n>> default (RCTRL[CFA]=0). When CFA=1, FCS is copied to the application\n>> buffer and bd->length includes it.\n>>\n>> Previously QEMU always included FCS in bd->length regardless of CFA,\n>> causing guests using the default RCTRL to receive 4 extra bytes per\n>> frame, breaking protocols that validate frame length (e.g. ARP).\n>>\n>> Fix by subtracting 4 from bd->length when RCTRL[CFA]=0 (default).\n>>\n>> Signed-off-by: Vivien LEGER \n>> ---\n>> hw/net/fsl_etsec/registers.h | 1 +\n>> hw/net/fsl_etsec/rings.c | 10 ++++++++++\n>> 2 files changed, 11 insertions(+)\n>>\n>> diff --git a/hw/net/fsl_etsec/registers.h b/hw/net/fsl_etsec/registers.h\n>> index f085537ecd..1766e39a1a 100644\n>> --- a/hw/net/fsl_etsec/registers.h\n>> +++ b/hw/net/fsl_etsec/registers.h\n>> @@ -103,6 +103,7 @@ extern const eTSEC_Register_Definition\n>> eTSEC_registers_def[];\n>> #define RCTRL_PRSDEP_MASK (0x3)\n>> #define RCTRL_PRSDEP_OFFSET (6)\n>> #define RCTRL_RSF (1 << 2)\n>> +#define RCTRL_CFA (1 << 11) /* Copy FCS to application */\n>>\n>> /* Index of each register */\n>>\n>> diff --git a/hw/net/fsl_etsec/rings.c b/hw/net/fsl_etsec/rings.c\n>> index 22660c32b8..38e27ad6d8 100644\n>> --- a/hw/net/fsl_etsec/rings.c\n>> +++ b/hw/net/fsl_etsec/rings.c\n>> @@ -442,6 +442,16 @@ static void fill_rx_bd(eTSEC *etsec,\n>> cpu_physical_memory_write(bufptr, padd, rem);\n>> }\n>> }\n>> +\n>> + /* Strip FCS unless RCTRL[CFA] requests copy to application.\n>> + * Per FSL eTSEC architecture (RCTRL field descriptions):\n>> + * CFA=0 (reset default): FCS is checked and discarded by hardware,\n>> + * bd->length should not include the 4-byte FCS.\n>> + * CFA=1: FCS is copied to the application buffer, bd->length\n>> includes FCS.\n>> + */\n>> + if (!(etsec->regs[RCTRL].value & RCTRL_CFA) && bd->length >= 4) {\n>> + bd->length -= 4;\n>> + }\n>> }\n>>\n>> static void rx_init_frame(eTSEC *etsec, const uint8_t *buf, size_t size)\n>> --\n>> 2.53.0\n>>\n>>","headers":{"Return-Path":"","X-Original-To":"incoming@patchwork.ozlabs.org","Delivered-To":"patchwork-incoming@legolas.ozlabs.org","Authentication-Results":["legolas.ozlabs.org;\n\tdkim=pass (2048-bit key;\n unprotected) header.d=gmail.com header.i=@gmail.com header.a=rsa-sha256\n header.s=20251104 header.b=i1LEPfcB;\n\tdkim-atps=neutral","legolas.ozlabs.org;\n spf=pass (sender SPF authorized) smtp.mailfrom=nongnu.org\n (client-ip=209.51.188.17; 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Sun, 12 Apr 2026\n 08:36:10 -0700 (PDT)","MIME-Version":"1.0","References":"<20260412134016.1640882-1-vivien.leger@gmail.com>\n ","In-Reply-To":"\n ","From":"Vivien LEGER ","Date":"Sun, 12 Apr 2026 17:35:57 +0200","X-Gm-Features":"AQROBzD3fcPTkgOLrNnf1kMY65xRihrVEOs6s_BwfTOfHjn6K4Q39YzOEqQ6UXk","Message-ID":"\n ","Subject":"Re: [PATCH] hw/net/fsl_etsec: implement RCTRL[CFA] FCS stripping on\n receive","To":"qemu-ppc@nongnu.org","Content-Type":"multipart/alternative; boundary=\"0000000000001911df064f45207f\"","Received-SPF":"pass client-ip=2607:f8b0:4864:20::b12c;\n envelope-from=vivien.leger@gmail.com; helo=mail-yx1-xb12c.google.com","X-Spam_score_int":"-20","X-Spam_score":"-2.1","X-Spam_bar":"--","X-Spam_report":"(-2.1 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1,\n DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, FREEMAIL_FROM=0.001,\n HTML_MESSAGE=0.001, RCVD_IN_DNSWL_NONE=-0.0001, SPF_HELO_NONE=0.001,\n SPF_PASS=-0.001 autolearn=ham autolearn_force=no","X-Spam_action":"no action","X-BeenThere":"qemu-ppc@nongnu.org","X-Mailman-Version":"2.1.29","Precedence":"list","List-Id":"","List-Unsubscribe":",\n ","List-Archive":"","List-Post":"","List-Help":"","List-Subscribe":",\n ","Errors-To":"qemu-ppc-bounces+incoming=patchwork.ozlabs.org@nongnu.org","Sender":"qemu-ppc-bounces+incoming=patchwork.ozlabs.org@nongnu.org"}}]