linux-kernel - Re: [PATCH v2 2/3] scsi: ufs: Modulize ufs-bsg

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Date:   Tue, 17 Dec 2019 16:56:23 +0800
From:   cang@...eaurora.org
To:     Bart Van Assche <bvanassche@....org>
Cc:     Bjorn Andersson <bjorn.andersson@...aro.org>,
        asutoshd@...eaurora.org, nguyenb@...eaurora.org,
        rnayak@...eaurora.org, linux-scsi@...r.kernel.org,
        kernel-team@...roid.com, saravanak@...gle.com, salyzyn@...gle.com,
        Alim Akhtar <alim.akhtar@...sung.com>,
        Avri Altman <avri.altman@....com>,
        Pedro Sousa <pedrom.sousa@...opsys.com>,
        "James E.J. Bottomley" <jejb@...ux.ibm.com>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        Evan Green <evgreen@...omium.org>,
        Kishon Vijay Abraham I <kishon@...com>,
        Stephen Boyd <swboyd@...omium.org>,
        Stanley Chu <stanley.chu@...iatek.com>,
        Vignesh Raghavendra <vigneshr@...com>,
        Bean Huo <beanhuo@...ron.com>,
        Venkat Gopalakrishnan <venkatg@...eaurora.org>,
        Tomas Winkler <tomas.winkler@...el.com>,
        Arnd Bergmann <arnd@...db.de>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 2/3] scsi: ufs: Modulize ufs-bsg

On 2019-12-17 01:22, Bart Van Assche wrote:
> On 12/15/19 8:36 PM, cang@...eaurora.org wrote:
>> On 2019-12-16 05:49, Bart Van Assche wrote:
>>> On 2019-12-11 22:37, Bjorn Andersson wrote:
>>>> It's the asymmetry that I don't like.
>>>> 
>>>> Perhaps if you instead make ufshcd platform_device_register_data() 
>>>> the
>>>> bsg device you would solve the probe ordering, the remove will be
>>>> symmetric and module autoloading will work as well (although then 
>>>> you
>>>> need a MODULE_ALIAS of platform:device-name).
>>> 
>>> Hi Bjorn,
>>> 
>>> From Documentation/driver-api/driver-model/platform.rst:
>>> "Platform devices are devices that typically appear as autonomous
>>> entities in the system. This includes legacy port-based devices and
>>> host bridges to peripheral buses, and most controllers integrated
>>> into system-on-chip platforms.  What they usually have in common
>>> is direct addressing from a CPU bus.  Rarely, a platform_device will
>>> be connected through a segment of some other kind of bus; but its
>>> registers will still be directly addressable."
>>> 
>>> Do you agree that the above description is not a good match for the
>>> ufs-bsg kernel module?
>> 
>> I missed this one.
>> How about making it a plain device and add it from ufs driver?
> 
> Hi Can,
> 
> Since the ufs_bsg kernel module already creates one device node under
> /dev/bsg for each UFS host I don't think that we need to create any
> additional device nodes for ufs-bsg devices. My proposal is to modify
> the original patch 2/3 from this series as follows:
> * Use module_init() instead of late_initcall_sync().
> * Remove the ufshcd_get_hba_list_lock() and
>   ufshcd_put_hba_list_unlock() functions.
> * Implement a notification mechanism in the UFS core that invokes a
>   callback function after an UFS host has been created and also after 
> an
>   UFS host has been removed.
> * Register for these notifications from inside the ufs-bsg driver.
> * During registration for notifications, invoke the UFS host creation
>   callback function for all known UFS hosts.
> * If the UFS core is unloaded, invoke the UFS host removal callback
>   function for all known UFS hosts.
> 
> I think there are several examples of similar notification mechanisms
> in the Linux kernel, e.g. the probe and remove callback functions in
> struct pci_driver.
> 
> Bart.

Hi Bart,

Even in the current ufs_bsg.c, it creates two devices, one is ufs-bsg,
one is the char dev node under /dev/bsg. Why this becomes a problem
after make it a module?

I took a look into the pci_driver, it is no different than making 
ufs-bsg
a plain device. The only special place about pci_driver is that it has 
its
own probe() and remove(), and the probe() in its bus_type calls the
probe() in pci_driver. Meaning the bus->probe() is an intermediate call
used to pass whatever needed by pci_driver->probe().

Of course we can also do this, but isn't it too much for ufs-bsg?
For our case, calling set_dev_drvdata(bsg_dev, hba) to pass hba to
ufs_bsg.c would be enough.

If you take a look at the V3 patch, the change makes the ufs_bsg.c
much conciser. platform_device_register_data() does everything for us,
initialize the device, set device name, provide the match func,
bus type and release func.

Since ufs-bsg is somewhat not a platform device, we can still add it
as a plain device, just need a few more lines to get it initialized.
This allows us leverage kernel's device driver model. Just like Greg
commented, we don't need to re-implement the mechanism again.

Thanks,
Can Guo.