From mboxrd@z Thu Jan 1 00:00:00 1970 From: Christer Weinigel Subject: speed_hz vs max_speed_hz Date: Wed, 15 Jul 2009 13:22:07 +0200 Message-ID: <4A5DBBDF.2050901@weinigel.se> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: spi-devel-general-5NWGOfrQmneRv+LV9MX5uipxlwaOVQ5f@public.gmane.org Return-path: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: spi-devel-general-bounces-5NWGOfrQmneRv+LV9MX5uipxlwaOVQ5f@public.gmane.org List-Id: linux-spi.vger.kernel.org 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 ------------------------------------------------------------------------------ Enter the BlackBerry Developer Challenge This is your chance to win up to $100,000 in prizes! For a limited time, vendors submitting new applications to BlackBerry App World(TM) will have the opportunity to enter the BlackBerry Developer Challenge. See full prize details at: http://p.sf.net/sfu/Challenge