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Message-ID: <20251028114746.GA1505797@bhelgaas>
Date: Tue, 28 Oct 2025 06:47:46 -0500
From: Bjorn Helgaas <helgaas@...nel.org>
To: Siddharth Vadapalli <s-vadapalli@...com>
Cc: kernel test robot <lkp@...el.com>, lpieralisi@...nel.org,
kwilczynski@...nel.org, mani@...nel.org, robh@...nel.org,
bhelgaas@...gle.com, unicorn_wang@...look.com, kishon@...nel.org,
18255117159@....com, oe-kbuild-all@...ts.linux.dev,
linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
linux-arm-kernel@...ts.infradead.org, srk@...com
Subject: Re: [PATCH] PCI: cadence: Enable support for applying lane
equalization presets
On Tue, Oct 28, 2025 at 10:56:33AM +0530, Siddharth Vadapalli wrote:
> On Tue, 2025-10-28 at 13:16 +0800, kernel test robot wrote:
> > Hi Siddharth,
> >
> > kernel test robot noticed the following build warnings:
> >
> > [auto build test WARNING on pci/next]
> > [also build test WARNING on pci/for-linus linus/master v6.18-rc3 next-20251027]
> > [If your patch is applied to the wrong git tree, kindly drop us a note.
> > And when submitting patch, we suggest to use '--base' as documented in
> > https://git-scm.com/docs/git-format-patch#_base_tree_information]
> >
> > url: https://github.com/intel-lab-lkp/linux/commits/Siddharth-Vadapalli/PCI-cadence-Enable-support-for-applying-lane-equalization-presets/20251027-213657
> > base: https://git.kernel.org/pub/scm/linux/kernel/git/pci/pci.git next
> > patch link: https://lore.kernel.org/r/20251027133013.2589119-1-s-vadapalli%40ti.com
> > patch subject: [PATCH] PCI: cadence: Enable support for applying lane equalization presets
> > config: x86_64-buildonly-randconfig-002-20251028 (https://download.01.org/0day-ci/archive/20251028/202510281329.racaZPSI-lkp@intel.com/config)
> > compiler: gcc-14 (Debian 14.2.0-19) 14.2.0
> > reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20251028/202510281329.racaZPSI-lkp@intel.com/reproduce)
> >
> > If you fix the issue in a separate patch/commit (i.e. not just a new version of
> > the same patch/commit), kindly add following tags
> > > Reported-by: kernel test robot <lkp@...el.com>
> > > Closes: https://lore.kernel.org/oe-kbuild-all/202510281329.racaZPSI-lkp@intel.com/
> >
> > All warnings (new ones prefixed by >>):
> >
> > drivers/pci/controller/cadence/pcie-cadence-host.c: In function 'cdns_pcie_setup_lane_equalization_presets':
> > > > drivers/pci/controller/cadence/pcie-cadence-host.c:205:20: warning: this statement may fall through [-Wimplicit-fallthrough=]
> > 205 | if (presets_ngts[0] != PCI_EQ_RESV) {
> > | ^
> > drivers/pci/controller/cadence/pcie-cadence-host.c:225:9: note: here
> > 225 | case PCIE_SPEED_8_0GT:
> > | ^~~~
>
> Fallthrough is intentional. The lane equalization presets are programmed
> starting from the Max Supported Link speed and we fallthrough until we get
> to 8.0 GT/s.
It's poorly documented, but use "fallthrough" here:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/process/deprecated.rst?id=v6.17#n200
> > vim +205 drivers/pci/controller/cadence/pcie-cadence-host.c
> >
> > 170
> > 171 static void cdns_pcie_setup_lane_equalization_presets(struct cdns_pcie_rc *rc)
> > 172 {
> > 173 struct cdns_pcie *pcie = &rc->pcie;
> > 174 struct device *dev = pcie->dev;
> > 175 struct device_node *np = dev->of_node;
> > 176 int max_link_speed, max_lanes, ret;
> > 177 u32 lane_eq_ctrl_reg;
> > 178 u16 cap;
> > 179 u16 *presets_8gts;
> > 180 u8 *presets_ngts;
> > 181 u8 i, j;
> > 182
> > 183 ret = of_property_read_u32(np, "num-lanes", &max_lanes);
> > 184 if (ret)
> > 185 return;
> > 186
> > 187 /* Lane Equalization presets are optional, so error message is not necessary */
> > 188 ret = of_pci_get_equalization_presets(dev, &rc->eq_presets, max_lanes);
> > 189 if (ret)
> > 190 return;
> > 191
> > 192 max_link_speed = of_pci_get_max_link_speed(np);
> > 193 if (max_link_speed < 0) {
> > 194 dev_err(dev, "%s: link-speed unknown, skipping preset setup\n", __func__);
> > 195 return;
> > 196 }
> > 197
> > 198 /*
> > 199 * Setup presets for data rates including and upward of 8.0 GT/s until the
> > 200 * maximum supported data rate.
> > 201 */
> > 202 switch (pcie_link_speed[max_link_speed]) {
> > 203 case PCIE_SPEED_16_0GT:
> > 204 presets_ngts = (u8 *)rc->eq_presets.eq_presets_Ngts[EQ_PRESET_TYPE_16GTS - 1];
> > > 205 if (presets_ngts[0] != PCI_EQ_RESV) {
> > 206 cap = cdns_pcie_find_ext_capability(pcie, PCI_EXT_CAP_ID_PL_16GT);
> > 207 if (!cap)
> > 208 break;
> > 209 lane_eq_ctrl_reg = cap + PCI_PL_16GT_LE_CTRL;
> > 210 /*
> > 211 * For Link Speeds including and upward of 16.0 GT/s, the Lane Equalization
> > 212 * Control register has the following layout per Lane:
> > 213 * Bits 0-3: Downstream Port Transmitter Preset
> > 214 * Bits 4-7: Upstream Port Transmitter Preset
> > 215 *
> > 216 * 'eq_presets_Ngts' is an array of u8 (byte).
> > 217 * Therefore, we need to write to the Lane Equalization Control
> > 218 * register in units of bytes per-Lane.
> > 219 */
> > 220 for (i = 0; i < max_lanes; i++)
> > 221 cdns_pcie_rp_writeb(pcie, lane_eq_ctrl_reg + i, presets_ngts[i]);
> > 222
> > 223 dev_info(dev, "Link Equalization presets applied for 16.0 GT/s\n");
> > 224 }
> > 225 case PCIE_SPEED_8_0GT:
> > 226 presets_8gts = (u16 *)rc->eq_presets.eq_presets_8gts;
> > 227 if ((presets_8gts[0] & PCI_EQ_RESV) != PCI_EQ_RESV) {
> > 228 cap = cdns_pcie_find_ext_capability(pcie, PCI_EXT_CAP_ID_SECPCI);
> > 229 if (!cap)
> > 230 break;
> > 231 lane_eq_ctrl_reg = cap + PCI_SECPCI_LE_CTRL;
> > 232 /*
> > 233 * For a Link Speed of 8.0 GT/s, the Lane Equalization Control register has
> > 234 * the following layout per Lane:
> > 235 * Bits 0-3: Downstream Port Transmitter Preset
> > 236 * Bits 4-6: Downstream Port Receiver Preset Hint
> > 237 * Bit 7: Reserved
> > 238 * Bits 8-11: Upstream Port Transmitter Preset
> > 239 * Bits 12-14: Upstream Port Receiver Preset Hint
> > 240 * Bit 15: Reserved
> > 241 *
> > 242 * 'eq_presets_8gts' is an array of u16 (word).
> > 243 * Therefore, we need to write to the Lane Equalization Control
> > 244 * register in units of words per-Lane.
> > 245 */
> > 246 for (i = 0, j = 0; i < max_lanes; i++, j += 2)
> > 247 cdns_pcie_rp_writew(pcie, lane_eq_ctrl_reg + j, presets_8gts[i]);
> > 248
> > 249 dev_info(dev, "Link Equalization presets applied for 8.0 GT/s\n");
> > 250 }
> > 251 }
> > 252 }
> > 253
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