sd_overclock does not work for EMMC5.1 cards requesting 52MHz

[ss8688]

Describe the bug

I used an EMMC5.1 to Micro SD converter on my raspberry pi 3b+, but sd_overclock doesn't work because EMMC5.1 requested 52MHz instead of 50MHz.EMMC5.1 will request 52MHz to traditional SD hosts.I hope to be able to use sd_overclock normally when using EMMC5.1

Device (s)

Raspberry Pi 3 Mod. B+

Logs

$ sudo cat /sys/kernel/debug/mmc0/ios
clock:          52000000 Hz
actual clock:   50000000 Hz
vdd:            21 (3.3 ~ 3.4 V)
bus mode:       2 (push-pull)
chip select:    0 (don't care)
power mode:     2 (on)
bus width:      2 (4 bits)
timing spec:    1 (mmc high-speed)
signal voltage: 0 (3.30 V)
driver type:    0 (driver type B)

Additional context

No response

[ss8688]

The problem lies in this line of code https://github.com/raspberrypi/linux/blob/78fd5c93f1568d0dac7c56ee3d497739a63b058e/drivers/mmc/host/bcm2835-mmc.c#L1071C2-L1072C49

[P33M]

You're reading the wrong driver. bcm2835-sdhost is used to drive the SD slot on 3B+. Even so, in both drivers, overclocking is possible, and it's a requirement that the card clock must be an even divisor of the source clock (see further down in the file).

In this case, that's 400MHz.
/sys/kernel/debug/clk/clk_summary

                                 enable  prepare  protect                                duty  hardware                            connection
   clock                          count    count    count        rate   accuracy phase  cycle    enable   consumer                         id
---------------------------------------------------------------------------------------------------------------------------------------------
 [...]
 osc                                 7       7        1        19200000    0          0     50000      Y   deviceless                      no_connection_id
 [...]
    pllc                             3       3        1        2400000000  0          0     50000      Y      deviceless                      no_connection_id
       pllc_core0                    2       2        1        1200000000  0          0     50000      Y         deviceless                      no_connection_id
          vpu                        2       2        1        400000000   0          0     50000      Y            3f805000.i2c                    no_connection_id
                                                                                                                    3f201000.serial                 apb_pclk
                                                                                             --->                   3f202000.mmc                    no_connection_id
                                                                                                                    3f215000.aux                    no_connection_id

Therefore requesting 52MHz will round down to 50MHz. The next step up would be 66.7MHz.

[pelwell]

Yes, @P33M, but the problem here is that the trigger doesn't work unless exactly 50MHz is requested. It needs some form of inequality or other method of determining that the full speed is required.

[ss8688]

@P33M Yes,but both drivers have this issue.

linux/drivers/mmc/host/bcm2835-sdhost.c

Line 1544 in 78fd5c9

if (host->overclock_50 && (clock == 50*MHZ))

[ss8688]

I have another question. My EMMC5.1 card can run at 200MHz. What is the maximum frequency that BCM2837's sdhost can support overclocking to? Is it possible to run at 200MHz (400MHz/2) or 134MHz (400MHz/3) after this issue is fixed?If possible, this is faster than the DDR50 speed of Raspberry Pi 4.

[pelwell]

Your mileage may vary, but I don't think I've seen it work reliably above 100MHz (that's with a divisor of 4). The next possible clock is 133MHz (a divisor of 3), which is a big jump.

N.B. To maintain that speed you will need to keep the core/peripheral clock at 400MHz, preventing it from dropping down to 250MHz. You can do this with core_freq=400 or, with a recent firmware, core_freq_fixed=1.