Thursday 22 November 2018

Tool for easily transferring files to/from SD card via serial

The MEGA65 uses an SD card as its bulk data storage device.  The ROMs, disk images, utility programs and other bits and pieces are all loaded from the SD card.  This means that while we are developing the MEGA65 there is often need to put updated files on the SD card, or to pull files from the SD card, so that we can check that things have been written properly.

This is a bit of a nuisance, as it means pulling the SD card, finding where on earth I have left my SD card adaptor (neither of my laptops have working SD card slots), and generally fiddling with things a lot.  For the MEGA65 rev1 PCB prototype, this is especially annoying, because the SD card interface on those is quite fragile, being hacked on after the PCBs were produced.

So, I have been thinking for a while of how to make an easier way to get files on and off the SD card, without having to pull the SD card out.  We have some progress on an FTP server for the MEGA65 that would allow this to happen at high-speed via ethernet, but that still requires having an ethernet connection, which isn't always the case, and won't be possible at all for the MEGA65 phone prototypes, since they won't have ethernet at all.

I finally realised that a simple way to solve this problem would be to make a program under linux that operates a lot like FTP, but actually communicates to the MEGA65 via the serial interface, and basically takes remote control of the SD card interface by stopping the CPU and writing directly to the IO registers for it, and using the serial monitor commands to read back the data.  This would require implementing a simple FAT file system reader, but I have already done that a few times.  Otherwise, the main issue is that the maximum throughput would be limited to 20KB/sec because of the 2mbit serial speed.  So nowhere near as fast as ethernet, but fast enough for easy fiddling --  especially since it can talk over the existing serial link, which means I can include accessing the SD card as part of automated tests.  So all up, it should be a nice solution.

The particular catalyst for getting around to this, is that I am tracking down a bug with writing to D81 disk images, and so have needed to do a lot of putting-back-the-unmodified-D81 followed by copying-back-the-one-I-just-broke, so that I could examine what the problem was.

So after a few hours of fiddling, I got to the point where I could parse a FAT32 directory (short names only for now), and find the cluster chain for a given file.  A few more hours, and I had cooked up something that is quite similar to FTP to use, but indeed works over the MEGA65 serial monitor interface.

I discovered or implemented a few interesting things in the process of this:

1. Sending serial commands back and forth by USB has a LOT of latency. I had hit this in other projects, but the combined latency of lots of commands was a big problem here. So I worked out how to set the USB serial port latency down from 16ms to 1ms.  That got the actual latency down to only about 3ms -- still 3x longer than it should be, but at least it helped a lot.

2. There are two ways to reduce the latency.  One uses the ASYNC_LOW_LATENCY IOCTL, and in theory doesn't require root access.  Sounds great, but doesn't actually seem to reduce the latency.  The other way is to use a command like: echo 1 | sudo tee /sys/bus/usb-serial/devices/ttyUSB0/latency_timer which does work, but requires root access. Very annoying.  The difference in performance is about 3x - 4x actual throughput, so it really makes a difference.

3. I can reliably set the serial speed to 4mbit/sec, instead of 2mbit/sec. That helps quite a lot with throughput, and lets us get close to 30KB/sec, which while not stellar, is good enough when pushing files <1MB in size.

4. Writing sectors is much slower than reading them, so when writing a sector, I first read to see if it has the correct value, and if so, don't write it.  This, together with a read-cache helps a lot to avoid unnecessarily re-reading sectors of the FAT a lot.

So how does it all look?  Here is an example session of me pushing a fresh D81 file:

$ src/tools/mega65_ftp -b bin/nexys4ddr-widget.bit
fpgajtag -a bin/nexys4ddr-widget.bit
fpgajtag: Digilent:Digilent USB Device:210292645477; bcd:700
count 0/1 cortex -1 dcount 0 trail 0
STATUS 0050107d done 0 release_done 0 eos 10 startup_state 4
fpgajtag: Starting to send file
fpgajtag: Done sending file
fpgajtag: bypass already programmed bc
STATUS 0050107d done 0 release_done 0 eos 10 startup_state 4
fpgajtag: ERROR failed to run pciescanportal: No such file or directory
[T+3sec] Bitstream loaded
Detected new-style UART monitor.
SD card not yet ready, so reset it.
MEGA65 SD-card> dir
Found FAT32 partition in partition slot 1 : start=0x800, size=965 MB
FAT32 file system has 965MB formatted capacity, first cluster = 2, 1928 sectors per FAT
FATs begin at sector 0x238 and 0x9c0
           0 M.E.G.A..65!
      131072 MEGA65.ROM
      819200 MEGA65.D81
MEGA65 SD-card> put ../c65/games2.d81 MEGA65.D81
Uploaded 819200 bytes in 28 seconds (28.6KB/sec)
MEGA65 SD-card>

You might notice that you have to supply a bitstream for the FPGA.  This is because we need to catch the MEGA65 booting in the Hypervisor, and stop the CPU there, so that we have full access over the hardware. Also, it makes sure we actually have a MEGA65 system to connect to, in case the FPGA board was previously doing something else.

So with this, I was able to work out that sector writes were only writing every other byte, fix that, and then find some rotated-by-1-byte bugs and fix those, all without having to workout where I have put that SD card adapter.  This helped me to fix the nasty disk corruption bug I was facing, that would cause sad scenes like this:

I'm now looking forward to beginning to write the freeze menu, as this really is the heart of the MEGA65 system in many ways, for selecting disk images to attach, as well as swapping running programs and so on.

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