[PATCH net-next] net: stmmac: qcom-ethqos: set clk_csr

["Russell King (Oracle)" <rmk+kernel@armlinux.org.uk>]

The clocks for qcom-ethqos return a rate of zero as firmware manages
their rate. According to hardware documentation, the clock which is
fed to the slave AHB interface can range between 50 to 100MHz for
non-RGMII and 30 to 75MHz for boards with a RGMII interfaces.

Currently, stmmac uses an undefined divisor value. Instead, use
STMMAC_CSR_60_100M which will mean we meet IEEE 802.3 specification
since this will generate:

  714kHz  @ 30MHz
  1.19MHz @ 50MHz
  1.79MHz @ 75MHz
  2.42MHz @ 100MHz

This gives MDC rates within the IEEE 802.3 specification, although the
30MHz case is particularly slow.

Selecting the next lowest divisor, STMMAC_CSR_35_60M, which is /26
will give:

   1.15MHz @ 30MHz
   1.92MHz @ 50MHz
   2.88MHz @ 75MHz (exceeding 802.3 spec)
   3.85MHz @ 100MHz (exceeding 802.3 spec)

Unfortunately, this divisor makes the upper bound of both ranges exeed
the IEEE 802.3 specification, and thus we can not use it without knowing
for certain what the current CSR clock rate actually is.

So, STMMAC_CSR_60_100M is the best fit for all boards based on the
information provided thus far.

Link: https://lore.kernel.org/r/acGhQ0oui+dVRdLY@oss.qualcomm.com
Link: https://lore.kernel.org/r/acw1habUsiSqlrky@oss.qualcomm.com
Reviewed-by: Mohd Ayaan Anwar <mohd.anwar@oss.qualcomm.com>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
---
 drivers/net/ethernet/stmicro/stmmac/dwmac-qcom-ethqos.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/drivers/net/ethernet/stmicro/stmmac/dwmac-qcom-ethqos.c b/drivers/net/ethernet/stmicro/stmmac/dwmac-qcom-ethqos.c
index ad3a983d2a08..ac7d6d3e205a 100644
--- a/drivers/net/ethernet/stmicro/stmmac/dwmac-qcom-ethqos.c
+++ b/drivers/net/ethernet/stmicro/stmmac/dwmac-qcom-ethqos.c
@@ -764,6 +764,12 @@ static int qcom_ethqos_probe(struct platform_device *pdev)
 	qcom_ethqos_set_sgmii_loopback(ethqos, true);
 	ethqos_set_func_clk_en(ethqos);
 
+	/* The clocks are controlled by firmware, so we don't know for certain
+	 * what clock rate is being used. Hardware documentation mentions that
+	 * the AHB slave clock will be in the range of 50 to 100MHz, which
+	 * equates to a MDC between 1.19 and 2.38MHz.
+	 */
+	plat_dat->clk_csr = STMMAC_CSR_60_100M;
 	plat_dat->bsp_priv = ethqos;
 	plat_dat->set_clk_tx_rate = ethqos_set_clk_tx_rate;
 	plat_dat->dump_debug_regs = rgmii_dump;
-- 
2.47.3