On 11/9/20 2:19 PM, Serge Semin wrote: > On Mon, Nov 09, 2020 at 02:14:22PM +0000, Mark Brown wrote: >> On Mon, Nov 09, 2020 at 08:47:10AM -0500, Sean Anderson wrote: >>> On 11/9/20 8:29 AM, Mark Brown wrote: >>>> On Sat, Nov 07, 2020 at 05:13:51PM +0900, Damien Le Moal wrote: >>>> > >>>>> The resting state of MOSI is high when nothing is driving it. If we >>>>> drive it low while recieving, it looks like we are transmitting 0x00 >>>>> instead of transmitting nothing. This can confuse slaves (like SD cards) >>>>> which allow new commands to be sent over MOSI while they are returning >>>>> data over MISO. The return of MOSI from 0 to 1 at the end of recieving >>>>> a byte can look like a start bit and a transmission bit to an SD card. > > Yeah, that's what we've also experienced on our systems. We've worked > around the problem in exactly the same way as you have. But we haven't > dared to send it out as the solution seemed a bit hackish. Well, the way it is now is equally wrong, since it is driving the line low. >>>> >>>> If client devices are interpreting the transmitted data then I would >>>> expect the drivers for that hardware to be ensuring that whatever we >>>> transmit matches what the device is expecting. We shouldn't be putting >>>> a hack in a particular controller driver to paper over things, that will >>>> mean that the device will break when used with other controllers and if >>>> different devices have different requirements then obviously we can't >>>> satisfy them. There is not meaningfully a general specification for SPI >>>> which says what happens when signals are idle, it's all specific to the >>>> client device. >>>> >>>> In this case it also looks like the controller hardware requires >>>> transmit data and therefore should be setting SPI_MUST_TX and just >>>> removing the in driver default anyway, though that will have no effect >>>> one way or anther on the issue you're seeing. >> > >>> There is a recieve-only mode, but it is not used by this driver. Perhaps >>> it should be. >> >> I'd expect it'd perform better, especially on systems that are >> apparently struggling for CPU bandwidth like yours seems to. > > CPU-wise. RO-mode won't help in that case. Moreover it will be even > more errors-prone for the systems with small CPU bandwidth. As I said > the Receive-only mode will make the SPI controller automatically > receiving data from the SPI bus and putting it into the Rx FIFO. If > CPU is either busy with something else or too slow in fetching the > data from the Rx FIFO, the FIFO will be eventually overflown with > data, which we need to avoid at all cost. > > As I see it the Receive-only mode is only acceptable in the next two > situations: > > 1) Rx-only DMA. But only if the DMA-engine and system bus are fast > enough to fetch the incoming data on time. (Note for example in our > system some DWC DMA-engine channels don't work well with the DW APB > SSI working with full-speed, so we had to set constraints on the DWC > DMA channels being used in conjunction with the DW APB SSI > controller.) > > 2) Rx-only with atomic CPU utilization. In order to make sure that the > CPU keeps up with fetching the data from the Rx FIFO, we have to > disable the local CPU IRQs while performing the Rx-only transfers, so > to prevent the Rx FIFO overflow while the CPU is doing something else. > Needless to say that such approach should be utilized only as a last > resort, if we have no choice but to run the Receive-only transfers. > Because locking the CPU for God knows how much time may cause the > system interactivity degradation. For instance, a possible use-case of > that design is when the controller is communicating with the > SPI-devices with native DW APB SSI chip-select attached. BTW You can > also find that design implemented in the kernel 5.10 spi-dw-core.c > driver in context of the SPI-memory operations (with my last patches > merged in). In particular I had to use it to handle the CPU-based > EEPROM-read mode. > > So in all other cases for normal CPU-based SPI-transfers when > GPIO-based chip-select is available the safest solution would be to > use a normal Push-Pull mode. In this case we have no risk in getting > the Rx FIFO overflow unless there is a bug in the code, which is > fixable anyway. > > Getting back to the patch. In fact I don't really see how the > Receive-only mode will help us with solving the problem noted in the > patch log. Shouldn't it put MOSI into High-Z like when the device is idle? The issue is mainly that the idle state and RX state are different. > As Mark said the problem with the Tx data on Rx-only > transfers should be fixed on the client side. If an subordinate > SPI-device needs a specific value to be received in that case, then > that value should be somehow provided to the SPI-controller anyway. > So the native Rx-only mode of the DW APB SSI controller won't help. > Currently it's possible to be done only by executing a Full-duplex > SPI-transfer with the Tx-buffer being pre-initialized with that > value. > > Another possible solution for the problem would be to fix the SPI core > so aside with tx_buf being set to the NULL-pointer, a client driver > would provide a default level or some specific value being put to the > SPI bus on Rx-only transfers. If an SPI-controller is capable of > satisfying the request, then it will accept the transfer. If it's not, > then the SPI core may try to convert the Rx-only transfer into the > Full-duplex transfer with the Tx-buffer being initialized with the > requested level. This is probably the most general solution; is there an existing way to specify this sort of thing? --Sean
