speed_hz vs max_speed_hz

speed_hz vs max_speed_hz

[Christer Weinigel]

Hi all,

I'm a bit confused about speed_hz and max_speed_hz.  How are they 
supposed to be used?

For example, on one board I have this:

static struct spi_board_info my_spi_board_info[] = {
         {
                 .mode           = 0,
                 .modalias       = "spidev",
                 .bus_num        = 0,
                 .irq            = IRQ_EINT00,
                 .max_speed_hz   = 4000000,
                 .chip_select    = S3C24A0_GP19,
         }
};

I have a chip connected to this bus that I know can run at 25MHz.  So 
what I would expect the above max_speed_hz to mean is that due to 
limitations of the SPI controller and the layout of the SPI wires on the 
board, even though the chip can run much faster, the maximum speed of 
this specific board is only 4MHz.

The driver I have right now sets spi_device->max_speed_hz to 25MHz 
because it "knows" that the chip can run that fast, so it will actually 
drive the chip much faster than the board supports.  Am I right in 
thinking that the driver is misbehaving and ought to set 
spi_transfer->speed_hz instead?

Also, what should the spi_setup do when passed a too high "max_speed"? 
If the SPI controller is only capable of 10MHz, what should spi_setup do 
if spi_device->max_speed_hz is 25MHz?  Should it return -EINVAL, 
silently change max_speed_hz to 10MHz, or set max_speed_hz to 25MHz and 
limit it to 10MHz when the transfer function is called?

I'd vote for changing max_speed_hz to 10MHz, since that would give the 
IMHO sanest ioctl interface, so that SPI_IOC_WR_MAX_SPEED_HZ call with a 
too high speed and a subsequent SPI_IOC_RD_MAX_SPEED_HZ would return the 
actual maximum speed.  In that case the debug printout in the write 
ioctl ought to be changed to:

     dev_dbg(&spi->dev, "%d Hz (max)\n", spi->max_speed_hz);

so that it shows the actual speed that was set.  Returning -EINVAL would 
also be a sane choice but slightly less useful.

No matter what, the ioctl should be documented as dangerous since it can 
set a clock which is faster than specified in the spi_board_info.

Or have I misunderstood everything? :-)

   /Christer


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Re: speed_hz vs max_speed_hz

[David Brownell]

On Wednesday 15 July 2009, Christer Weinigel wrote:
> Hi all,
> 
> I'm a bit confused about speed_hz and max_speed_hz.  How are they 
> supposed to be used?
> 
> For example, on one board I have this:
> 
> static struct spi_board_info my_spi_board_info[] = {
>          {
>                  .mode           = 0,
>                  .modalias       = "spidev",
>                  .bus_num        = 0,
>                  .irq            = IRQ_EINT00,
>                  .max_speed_hz   = 4000000,
>                  .chip_select    = S3C24A0_GP19,
>          }
> };
> 
> I have a chip connected to this bus that I know can run at 25MHz.  So 
> what I would expect the above max_speed_hz to mean is that due to 
> limitations of the SPI controller and the layout of the SPI wires on the 
> board, even though the chip can run much faster, the maximum speed of 
> this specific board is only 4MHz.

Exact.


> The driver I have right now sets spi_device->max_speed_hz to 25MHz 
> because it "knows" that the chip can run that fast, so it will actually 
> drive the chip much faster than the board supports.  Am I right in 
> thinking that the driver is misbehaving

Yes.


> and ought to set  
> spi_transfer->speed_hz instead?

That is less clear.  Controller drivers tend to use that
regardless, instead of applying the board-specific ceiling.
Given a choice I'd fix the controller driver, but it's
always OK for protocol drivers to apply it .....

That said, per-transfer speed overrides are a bit dicey.


> Also, what should the spi_setup do when passed a too high "max_speed"?
> If the SPI controller is only capable of 10MHz, what should spi_setup do 
> if spi_device->max_speed_hz is 25MHz?  Should it return -EINVAL, 
> silently change max_speed_hz to 10MHz, or set max_speed_hz to 25MHz and 
> limit it to 10MHz when the transfer function is called?

IMO the second option:  10MHz limiter.


> I'd vote for changing max_speed_hz to 10MHz, since that would give the 
> IMHO sanest ioctl interface, so that SPI_IOC_WR_MAX_SPEED_HZ call with a 
> too high speed and a subsequent SPI_IOC_RD_MAX_SPEED_HZ would return the 
> actual maximum speed.  In that case the debug printout in the write 
> ioctl ought to be changed to:
> 
>      dev_dbg(&spi->dev, "%d Hz (max)\n", spi->max_speed_hz);
> 
> so that it shows the actual speed that was set.  Returning -EINVAL would 
> also be a sane choice but slightly less useful.

Right.

 
> No matter what, the ioctl should be documented as dangerous since it can 
> set a clock which is faster than specified in the spi_board_info.

Not if it's limited...



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