mtd: rawnand: gpmi: validate controller clock rate

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 to set
edo mode 5 (100MHz) but the rate returned by clk_round_rate() is 80MHz
(edo mode 4). In this case gpmi_nfc_compute_timings() will return error,
and will be called again to set edo mode 4, which this time will be
successful.

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

Co-developed-by: Michael Trimarchi <michael@amarulasolutions.com>
Signed-off-by: Michael Trimarchi <michael@amarulasolutions.com>
Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>
Tested-by: Sascha Hauer <s.hauer@pengutronix.de>
Reviewed-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20220118095434.35081-4-dario.binacchi@amarulasolutions.com

drivers/mtd/nand/raw/gpmi-nand/gpmi-nand.c

@@ -644,8 +644,8 @@ static int bch_set_geometry(struct gpmi_nand_data *this)
  *         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;
@@ -657,23 +657,33 @@ static void gpmi_nfc_compute_timings(struct gpmi_nand_data *this,
 	int sample_delay_ps, sample_delay_factor;
 	u16 busy_timeout_cycles;
 	u8 wrn_dly_sel;
+	unsigned long clk_rate, min_rate;
 
 	if (sdr->tRC_min >= 30000) {
 		/* ONFI non-EDO modes [0-3] */
 		hw->clk_rate = 22000000;
+		min_rate = 0;
 		wrn_dly_sel = BV_GPMI_CTRL1_WRN_DLY_SEL_4_TO_8NS;
 	} else if (sdr->tRC_min >= 25000) {
 		/* ONFI EDO mode 4 */
 		hw->clk_rate = 80000000;
+		min_rate = 22000000;
 		wrn_dly_sel = BV_GPMI_CTRL1_WRN_DLY_SEL_NO_DELAY;
 	} else {
 		/* ONFI EDO mode 5 */
 		hw->clk_rate = 100000000;
+		min_rate = 80000000;
 		wrn_dly_sel = BV_GPMI_CTRL1_WRN_DLY_SEL_NO_DELAY;
 	}
 
-	hw->clk_rate = clk_round_rate(r->clock[0], hw->clk_rate);
+	clk_rate = clk_round_rate(r->clock[0], hw->clk_rate);
+	if (clk_rate <= min_rate) {
+		dev_err(this->dev, "clock setting: expected %ld, got %ld\n",
+			hw->clk_rate, clk_rate);
+		return -ENOTSUPP;
+	}
 
+	hw->clk_rate = clk_rate;
 	/* SDR core timings are given in picoseconds */
 	period_ps = div_u64((u64)NSEC_PER_SEC * 1000, hw->clk_rate);
 
@@ -714,6 +724,7 @@ 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)
@@ -769,6 +780,7 @@ 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);
@@ -784,7 +796,9 @@ 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;