[PATCH]irqchip/irq-gic-v3:Avoid a waste of LPI resource

[Zhang, Lei]

Hi Mark, Marc

> -----Original Message-----
> From: linux-arm-kernel
> [mailto:linux-arm-kernel-bounces at lists.infradead.org] On Behalf Of
> Mark Langsdorf
> Sent: Wednesday, May 09, 2018 11:32 PM
> To: linux-arm-kernel at lists.infradead.org
> Subject: Re: [PATCH]irqchip/irq-gic-v3:Avoid a waste of LPI resource
> 
 
> Marc's suggestion of implementing a glue layer, and then changing the
> glue layer interface to pass the allocation requirements up to this
> driver is the best solution that can be supported for server products.

Thanks for your comments.
 
I'm considering implementing a glue layer. But the management of 
whole LPI resource is implemented by core ITS driver (irq-gic-v3-its.c),
So I think core ITS driver also need to be modified.
 
Below is my idea for core ITS driver. 
Would you give me comments?
 
Add an interface that can specify the number of LPIs allocation requirements on Core ITS driver.
My idea is extend its_msi_prepare function by adding two argument.

SYNOPSIS
its_msi_prepare(struct irq_domain *domain, struct device *dev,int nvec,
 msi_alloc_info_t *info,int request_nr_lpis, int request_lpis_align)
 
Argument "request_nr_lpis" means request LPIs total number for device. 
Argument "request_lpis_align" means request of LPIs alignment. 0 means do not specify alignment.

For PCI, PCI glue layer specifies, request_nr_lpis = 32, request_lpis_align = 32.
For our device, the glue layer specifies, request_nr_lpis = 1, request_lpis_align = 0.
(We have lots of device but each device only need a single LPI on our original bus. )
 

For expanding two arguments, core ITS driver maybe need to discard chunk mechanism 
and use bitmap to manage LPI resource directly.
 
Example:
 (1) Non PCI glue layer requires “request_nr_lpis” = 1, “request_lpis_align” = 0.
 
    Bitmap will become  ...0000000000000000001
 
 (2) PCI glue layer requires “request_nr_lpis” = 32, “request_lpis_align” = 32.
 
    Bitmap will become  ...000FFFFFFFF00000001
 
 
 (3) Non PCI glue layer requires “request_nr_lpis” = 1, “request_lpis_align” = 0.
 
    Bitmap will become  ...000FFFFFFFF00000003
 
I think it may be easy to implement by using bitmap_find_next_zero_area, because it has “align_mask” argument.

Regards,
Lei Zhang
--
Lei Zhang  e-mail: zhang.lei at jp.fujitsu.com FUJITSU LIMITED