Re: [PATCH v9 24/27] dmaengine: dw-edma: Relax driver config settings

[Bjorn Helgaas <helgaas@kernel.org>]

On Sun, Jan 22, 2023 at 03:11:16AM +0300, Serge Semin wrote:
> On Fri, Jan 20, 2023 at 04:50:36PM -0600, Bjorn Helgaas wrote:
> > On Fri, Jan 13, 2023 at 08:14:06PM +0300, Serge Semin wrote:
> > > Since the DW PCIe RP/EP driver is about to be updated to register the DW
> > > eDMA-based DMA-engine the drivers build modes must be synchronized.
> > > Currently the DW PCIe RP/EP driver is always built as a builtin module.
> > > Meanwhile the DW eDMA driver can be built as a loadable module. Thus in
> > > the later case the kernel with DW PCIe controllers support will fail to be
> > > linked due to lacking the DW eDMA probe/remove symbols. At the same time
> > > forcibly selecting the DW eDMA driver from the DW PCIe RP/EP kconfig will
> > > effectively eliminate the tristate type of the former driver fixing it to
> > > just the builtin kernel module.
> > > 
> > > Seeing the DW eDMA engine isn't that often met built into the DW PCIe
> > > Root-ports and End-points let's convert the DW eDMA driver config to being
> > > more flexible instead of just forcibly selecting the DW eDMA kconfig. In
> > > order to do that first the DW eDMA PCIe driver config should be converted
> > > to being depended from the DW eDMA core config instead of selecting the
> > > one. Second the DW eDMA probe and remove symbols should be referenced only
> > > if they are reachable by the caller. Thus the user will be able to build
> > > the DW eDMA core driver with any type, meanwhile the dependent code will
> > > be either restricted to the same build type (e.g. DW eDMA PCIe driver if
> > > DW eDMA driver is built as a loadable module) or just won't be able to use
> > > the eDMA engine registration/de-registration functionality (e.g. DW PCIe
> > > RP/EP driver if DW eDMA driver is built as a loadable module).
> > 
> > I'm trying to write the merge commit log, and I understand the linking
> > issue, but I'm having a hard time figuring out what the user-visible
> > scenarios are here.
> > 
> > I assume there's something that works when CONFIG_PCIE_DW=y and
> > CONFIG_DW_EDMA_PCIE=y but does *not* work when CONFIG_PCIE_DW=y and
> > CONFIG_DW_EDMA_PCIE=m?
> 
> No. The DW eDMA code availability (in other words the CONFIG_DW_EDMA
> config value) determines whether the corresponding driver (DW PCIe
> RP/EP or DW eDMA PCI) is capable to perform the eDMA engine probe
> procedure. Additionally both drivers has the opposite dependency from
> the DW eDMA code.
> |                |     DW PCIe RP/EP    |     DW eDMA PCIe     |
> | CONFIG_DW_EDMA +----------------------+----------------------+
> |                | Probe eDMA | KConfig | Probe eDMA | Kconfig |
> +----------------+------------+---------+------------+---------+
> |        y       |     YES    |   y,n   |     YES    |  y,m,n  |
> |        m       |     NO     |   y,n   |     YES    |    m,n  |
> |        n       |     NO     |   y,n   |     NO     |      n  |
> +--------------------------------------------------------------+
> 
> Basically it means the DW PCIe RP/EP driver will be able to probe the
> DW eDMA engine only if the corresponding driver is built into the
> kernel. At the same time the DW PCIe RP/EP driver doesn't depend on
> the DW eDMA core module config state. The DW eDMA PCIe driver in
> opposite depends on the DW eDMA code config state, but will always be
> able to probe the DW eDMA engine as long as the corresponding code is
> loaded as either a part of the kernel or as a loadable module.
> 
> > If both scenarios worked the same, I would think the existing
> > dw_edma_pcie_probe() would be enough, and you wouldn't need to call
> > dw_pcie_edma_detect() from dw_pcie_host_init() and dw_pcie_ep_init().
> 
> No. These methods have been implemented for the absolutely different
> drivers.
> dw_edma_pcie_probe() is called for an end-point PCIe-device found on a
> PCIe-bus.
> dw_pcie_host_init()/dw_pcie_ep_init() and dw_pcie_edma_detect() are
> called for a platform-device representing a DW PCIe RP/EP controller.
> In other words dw_pcie_edma_detect() and dw_edma_pcie_probe() are in
> no means interchangeable.

The question is what the user-visible difference between
CONFIG_DW_EDMA_PCIE=y and CONFIG_DW_EDMA_PCIE=m is.  If there were no
difference, dw_pcie_host_init() would not need to call
dw_pcie_edma_detect().

Can you give me a one- or two-sentence merge commit comment that
explains why we want to merge this?  "Relax driver config settings"
doesn't tell us that.

Bjorn