[RFC PATCH v2 4/5] mtd: rawnand: gpmi: validate controller clock rate

[Dario Binacchi]

What to do when the real rate of the gpmi clock is not equal to the
required one? The solutions proposed in [1] did not lead to a conclusion
on how to validate the clock rate, so, inspired by the document [2], I
consider the rate correct only if not lower or equal to the rate of the
previous edo mode. In fact, in chapter 4.16.2 (NV-DDR) of the document [2],
it is written that "If the host selects timing mode n, then its clock
period shall be faster than the clock period of timing mode n-1 and
slower than or equal to the clock period of timing mode n.". I thought
that it could therefore also be used in this case, without therefore
having to define the valid rate ranges empirically.

For example, suppose that gpmi_nfc_compute_timings() is called for edo
mode 5 configuration (100MHz, from the `edo_modes' table) but the rate
returned by clk_round_rate() is 80MHz (edo mode 4 from the `edo_modes'
table). In this case gpmi_nfc_compute_timings() will return error, and
will be called again for edo mode 4 configuration, which this time will
be successful.

[1] https://lore.kernel.org/r/20210702065350.209646-5-ebiggers@xxxxxxxxxx
[2] http://www.onfi.org/-/media/client/onfi/specs/onfi_3_0_gold.pdf?la=en

Co-developed-by: Michael Trimarchi <michael@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Michael Trimarchi <michael@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Dario Binacchi <dario.binacchi@xxxxxxxxxxxxxxxxxxxx>

---

Changes in v2:
- Fix commit description.
- Add an example to the commit description to better understand the
  problem solved by the patch.
- Split the patch.

 drivers/mtd/nand/raw/gpmi-nand/gpmi-nand.c | 24 ++++++++++++++++++----
 1 file changed, 20 insertions(+), 4 deletions(-)

diff --git a/drivers/mtd/nand/raw/gpmi-nand/gpmi-nand.c b/drivers/mtd/nand/raw/gpmi-nand/gpmi-nand.c
index 4ac695aa5131..7ae7a37775a3 100644
--- a/drivers/mtd/nand/raw/gpmi-nand/gpmi-nand.c
+++ b/drivers/mtd/nand/raw/gpmi-nand/gpmi-nand.c
@@ -665,8 +665,8 @@ static const struct edo_mode edo_modes[] = {
  *         RDN_DELAY = -----------------------     {3}
  *                           RP
  */
-static void gpmi_nfc_compute_timings(struct gpmi_nand_data *this,
-				     const struct nand_sdr_timings *sdr)
+static int gpmi_nfc_compute_timings(struct gpmi_nand_data *this,
+				    const struct nand_sdr_timings *sdr)
 {
 	struct gpmi_nfc_hardware_timing *hw = &this->hw;
 	struct resources *r = &this->resources;
@@ -679,6 +679,7 @@ static void gpmi_nfc_compute_timings(struct gpmi_nand_data *this,
 	u16 busy_timeout_cycles;
 	u8 wrn_dly_sel;
 	int i, emode = ARRAY_SIZE(edo_modes) - 1;
+	long clk_rate;
 
 	/* Search the required EDO mode */
 	for (i = 0; i < ARRAY_SIZE(edo_modes); i++) {
@@ -688,7 +689,18 @@ static void gpmi_nfc_compute_timings(struct gpmi_nand_data *this,
 		}
 	}
 
-	hw->clk_rate = clk_round_rate(r->clock[0], edo_modes[emode].clk_rate);
+	clk_rate = clk_round_rate(r->clock[0], edo_modes[emode].clk_rate);
+	if (emode > 0 && !(clk_rate <= edo_modes[emode].clk_rate &&
+			   clk_rate > edo_modes[emode - 1].clk_rate)) {
+		dev_err(this->dev,
+			"edo mode %d clock setting: expected %ld, got %ld\n",
+			emode, edo_modes[emode].clk_rate, clk_rate);
+		return -ENOTSUPP;
+	}
+
+	dev_dbg(this->dev, "edo mode %d @ %ld Hz\n", emode, clk_rate);
+
+	hw->clk_rate = clk_rate;
 	wrn_dly_sel = edo_modes[emode].wrn_dly_sel;
 
 	/* SDR core timings are given in picoseconds */
@@ -731,6 +743,7 @@ static void gpmi_nfc_compute_timings(struct gpmi_nand_data *this,
 		hw->ctrl1n |= BF_GPMI_CTRL1_RDN_DELAY(sample_delay_factor) |
 			      BM_GPMI_CTRL1_DLL_ENABLE |
 			      (use_half_period ? BM_GPMI_CTRL1_HALF_PERIOD : 0);
+	return 0;
 }
 
 static int gpmi_nfc_apply_timings(struct gpmi_nand_data *this)
@@ -786,6 +799,7 @@ static int gpmi_setup_interface(struct nand_chip *chip, int chipnr,
 {
 	struct gpmi_nand_data *this = nand_get_controller_data(chip);
 	const struct nand_sdr_timings *sdr;
+	int ret;
 
 	/* Retrieve required NAND timings */
 	sdr = nand_get_sdr_timings(conf);
@@ -801,7 +815,9 @@ static int gpmi_setup_interface(struct nand_chip *chip, int chipnr,
 		return 0;
 
 	/* Do the actual derivation of the controller timings */
-	gpmi_nfc_compute_timings(this, sdr);
+	ret = gpmi_nfc_compute_timings(this, sdr);
+	if (ret)
+		return ret;
 
 	this->hw.must_apply_timings = true;
 
-- 
2.32.0