linux-kernel - drm/nouveau/bios/ramcfg, setting of RON pull value

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Message-ID: <2e3222ec-3355-b3d9-8409-5dcdc8b6de0c@canonical.com>
Date:   Sat, 16 Feb 2019 14:59:54 +0000
From:   Colin Ian King <colin.king@...onical.com>
To:     Roy Spliet <rspliet@...ipso.eu>, Ben Skeggs <bskeggs@...hat.com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        dri-devel@...ts.freedesktop.org, nouveau@...ts.freedesktop.org
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject:  drm/nouveau/bios/ramcfg, setting of RON pull value

Hi,

Static Analysis with CoverityScan as detected an issue with the setting
of the RON pull value in function nvkm_gddr3_calc in
drm/nouveau/bios/ramcfg.c

This was introduced by commit: c25bf7b6155cb ("drm/nouveau/bios/ramcfg:
Separate out RON pull value")

CoverityScan reports the issue as follows:

 84        case 0x20:
 85                CWL = (ram->next->bios.timing[1] & 0x00000f80) >> 7;
 86                CL  = (ram->next->bios.timing[1] & 0x0000001f) >> 0;
 87                WR  = (ram->next->bios.timing[2] & 0x007f0000) >> 16;
 88                /* XXX: Get these values from the VBIOS instead */
 89                DLL = !(ram->mr[1] & 0x1);

   CID 1324005 (#1 of 1): Operands don't affect result
(CONSTANT_EXPRESSION_RESULT)

result_independent_of_operands: !(ram->mr[1] & 768) >> 8 is 0 regardless
of the values of its operands. This occurs as the operand of assignment.

 90                RON = !(ram->mr[1] & 0x300) >> 8;
 91                break;

Looking at this, I believe perhaps the correct setting could be:

RON = !((ram->mr[1] & 0x300) >> 8);

..however I don't have the datasheet available for the H/W so I'm not
sure if this a correct fix.

Colin