linux-kernel - Re: [PATCH v4] Add new uio device for PCI with dynamic memory allocation

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Date:   Wed, 29 Apr 2020 13:53:02 +0000
From:   "Stahl, Manuel" <manuel.stahl@...-extern.fraunhofer.de>
To:     "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "devel@...verdev.osuosl.org" <devel@...verdev.osuosl.org>,
        "sojkam1@....cvut.cz" <sojkam1@....cvut.cz>
Subject: Re: [PATCH v4] Add new uio device for PCI with dynamic memory
 allocation

On Mi, 2020-04-29 at 11:41 +0200, gregkh@...uxfoundation.org wrote:
> On Wed, Apr 29, 2020 at 07:51:01AM +0000, Stahl, Manuel wrote:
> > On Di, 2020-04-28 at 15:54 +0200, gregkh @ linuxfoundation . org wrote:
> > > On Thu, Apr 16, 2020 at 06:38:30PM +0200, Manuel Stahl wrote:
> > > > 
> > > > + *
> > > > + * Since the driver does not declare any device ids, you must allocate
> > > > + * id and bind the device to the driver yourself.  For example:
> > > > + *
> > > > + * # echo "8086 10f5" > /sys/bus/pci/drivers/uio_pci_dmem_genirq/new_id
> > > > + * # echo -n 0000:00:19.0 > /sys/bus/pci/drivers/e1000e/unbind
> > > > + * # echo -n 0000:00:19.0 > /sys/bus/pci/drivers/uio_pci_dmem_genirq/bind
> > > > + * # ls -l /sys/bus/pci/devices/0000:00:19.0/driver
> > > > + * .../0000:00:19.0/driver -> ../../../bus/pci/drivers/uio_pci_dmem_genirq
> > > > + *
> > > > + * Or use a modprobe alias:
> > > > + * # alias pci:v000010EEd00001000sv*sd*sc*i* uio_pci_dmem_genirq
> > > > + *
> > > > + * Driver won't bind to devices which do not support the Interrupt Disable Bit
> > > > + * in the command register. All devices compliant to PCI 2.3 (circa 2002) and
> > > > + * all compliant PCI Express devices should support this bit.
> > > > + *
> > > > + * The DMA mask bits and sizes of dynamic regions are derived from module
> > > > + * parameters.
> > > > + *
> > > > + * The format for specifying dynamic region sizes in module parameters
> > > > + * is as follows:
> > > > + *
> > > > + * uio_pci_dmem_genirq.dmem_sizes := <uio_dmem_sizes_def>[;<uio_dmem_sizes_def>]
> > > > + * <uio_dmem_sizes_def>           := <pci_id>:<size>[,<size>]
> > > > + * <pci_id>                       := <vendor>:<device>
> > > > + * <size>                         := standard linux memsize
> > > > + *
> > > > + * Examples:
> > > > + *
> > > > + * 1) UIO dmem device with 3 dynamic regions:
> > > > + * uio_pci_dmem_genirq.dmem_sizes=8086:10f5:4K,16K,4M
> > > > + *
> > > > + * 2) Two UIO dmem devices with different number of dynamic regions:
> > > > + * uio_pci_dmem_genirq.dmem_sizes=8086:10f5:4K,16K,4M;1234:0001:8K
> > > 
> > > Module parameters are horrid, are you sure there is no other way?
> > 
> > You're right, seemed to be the simplest solution back when we started developing this driver. I will try to change it to sysfs, so that one can add regions while the module is already loaded.
> 
> /me hands you some \n characters...
> 
> Anyway, configfs is for configuring stuff, don't make a sysfs file that
> you have to somehow "parse" please.

Looking back at this driver after some years I realized again the reason
for using kernel parameters:

The current UIO API needs the information about available memory maps when
registering a new UIO device with __uio_register_device(), which obviously
needs to be called during probe() in uio_pci_dmem_genirq. Otherwise there
is no device file in /dev to open for user space applications.

After that there is no function to update the uio_map info. So we can either
keep the module parameters and allocate the DMA memory during probe() or
allocate the DMA memory during mmap() and
  a) replicate parts of uio_dev_add_attributes() in this driver to update sysfs
  b) add a function in uio.c to allow updates to the uio_map

Which way would you go?

Best regards,
Manuel Stahl