Re: [PATCH v18 04/20] PCI: dwc: Change arguments of dw_pcie_prog_outbound_atu()

[Manivannan Sadhasivam <mani@kernel.org>]

On Thu, Jul 27, 2023 at 02:38:44AM +0300, Serge Semin wrote:
> On Wed, Jul 26, 2023 at 06:30:15PM +0530, Manivannan Sadhasivam wrote:
> > On Wed, Jul 26, 2023 at 08:02:24AM +0300, Serge Semin wrote:
> > > On Mon, Jul 24, 2023 at 01:15:56PM +0530, Manivannan Sadhasivam wrote:
> > > > On Fri, Jul 21, 2023 at 04:44:36PM +0900, Yoshihiro Shimoda wrote:
> > > > > The __dw_pcie_prog_outbound_atu() currently has 6 arguments.
> > > > > To support INTx IRQs in the future, it requires an additional 2
> > > > > arguments. For improved code readability, introduce the struct
> > > > > dw_pcie_ob_atu_cfg and update the arguments of
> > > > > dw_pcie_prog_outbound_atu().
> > > > > 
> > > > > Consequently, remove __dw_pcie_prog_outbound_atu() and
> > > > > dw_pcie_prog_ep_outbound_atu() because there is no longer
> > > > > a need.
> > > > > 
> > > > > No behavior changes.
> > > > > 
> > > > > Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
> > > > 
> > > > One nit below. With that,
> > > > 
> > > > Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
> > > > 
> > > > > Reviewed-by: Serge Semin <fancer.lancer@gmail.com>
> > > > > ---
> > > > >  .../pci/controller/dwc/pcie-designware-ep.c   | 21 +++++---
> > > > >  .../pci/controller/dwc/pcie-designware-host.c | 52 +++++++++++++------
> > > > >  drivers/pci/controller/dwc/pcie-designware.c  | 49 ++++++-----------
> > > > >  drivers/pci/controller/dwc/pcie-designware.h  | 15 ++++--
> > > > >  4 files changed, 77 insertions(+), 60 deletions(-)
> > > > > 
> > > > 
> > > > [...]
> > > > 
> > > > > diff --git a/drivers/pci/controller/dwc/pcie-designware.h b/drivers/pci/controller/dwc/pcie-designware.h
> > > > > index 3c06e025c905..85de0d8346fa 100644
> > > > > --- a/drivers/pci/controller/dwc/pcie-designware.h
> > > > > +++ b/drivers/pci/controller/dwc/pcie-designware.h
> > > > > @@ -288,6 +288,15 @@ enum dw_pcie_core_rst {
> > > > >  	DW_PCIE_NUM_CORE_RSTS
> > > > >  };
> > > > >  
> > > > > +struct dw_pcie_ob_atu_cfg {
> > > > > +	int index;
> > > > > +	int type;
> > > > > +	u8 func_no;
> > > > > +	u64 cpu_addr;
> > > > > +	u64 pci_addr;
> > > > > +	u64 size;
> > > > 
> > > 
> > > > Reorder the members in below order to avoid holes:
> > > > 
> > > > u64
> > > > int
> > > > u8
> > > 
> > > One more time. Your suggestion won't prevent the compiler from adding
> > > the pads. (If by "holes" you meant the padding. Otherwise please
> > > elaborate what you meant?).
> > 
> > Struct padding is often referred as struct holes. So yes, I'm referring the
> > same.
> > 
> > > The structure will have the same size of
> > > 40 bytes in both cases. So your suggestion will just worsen the
> > > structure readability from having a more natural parameters order (MW
> > > index, type, function, and then the mapping parameters) to a redundant
> > > type-based order.
> > > 
> > 
> 
> > This is a common comment I provide for all structures. Even though the current
> > result (reordering) doesn't save any space, when the structure grows big (who
> > knows), we often see more holes/padding being inserted by the compiler if the
> > members are not ordered in the descending order w.r.t their size.
> > 
> > I agree that it makes more clear if the members are grouped based on their
> > function etc... but for large structures this would often add more padding/hole.
> 
> This structure will never be big enough to be considered for such
> strange optimization. Moreover practicality almost always beats some
> theoretical considerations. In this case there is no any reason to
> reorder the fields as you say.
> 
> Speaking in general I very much doubt that saving a few bytes of
> memory can be considered as a better option than having a more
> readable structure especially these days. Moreover for all these years
> I never met anybody asking to set the descending order of
> the members or maintaining such limitation in the commonly used kernel
> structures. What is normally done:
> 1. Move an embedded object to the head of the structure for the
> container_of-macro optimization.
> 2. Group up the commonly used fields to optimize the system cache
> utilization.
> 3. Logical grouping the members, which naturally may lead to the more
> optimal cache utilization.

Indeed.

> 4. Move a field to a certain place of the structure to fill in the
> pads.
> 

This is what I try to avoid by grouping the members. If you move a field to
a certain place, wouldn't it affect readability?

But I do not want to argue more on this. Please see below.

> Even if the "descending alignment" requirement minimizes the number of
> the pads it isn't the only possible way to do so in the particular
> cases and it looks too harsh to be blindly applied all the time. If a
> few bytes is so important why not do the same for instance for the
> local variables too? They are also normally size-aligned in the stack
> memory, which is much more precious in kernel.
> 

Well, for local variables I prefer reverse Xmas tree order which is what widely
used throughout the kernel. But we do not care about their ordering because, it
won't grow too much like a structure (not talking about recursive case).

> Anyway in this case changing the fields order is absolutely redundant.
> Even a provided afterwards update doesn't cause the structure size
> change. So for the sake of readability it's better to leave its fields
> ordered as is.
> 

I certainly agree that reordering wouldn't save any space for this structure.
As a maintainer, I prefer to keep this pattern so that I don't have to worry
about the padding issues in the future and hence the suggestion.

But feel free to drop it as I don't have a strong objection to this specific
case.

- Mani

> -Serge(y)
> 
> > 
> > - Mani
> > 
> > > -Serge(y)
> > > 
> > > > 
> > > > - Mani
> > > > 
> > > > > +};
> > > > > +
> > > > >  struct dw_pcie_host_ops {
> > > > >  	int (*host_init)(struct dw_pcie_rp *pp);
> > > > >  	void (*host_deinit)(struct dw_pcie_rp *pp);
> > > > > @@ -416,10 +425,8 @@ void dw_pcie_write_dbi2(struct dw_pcie *pci, u32 reg, size_t size, u32 val);
> > > > >  int dw_pcie_link_up(struct dw_pcie *pci);
> > > > >  void dw_pcie_upconfig_setup(struct dw_pcie *pci);
> > > > >  int dw_pcie_wait_for_link(struct dw_pcie *pci);
> > > > > -int dw_pcie_prog_outbound_atu(struct dw_pcie *pci, int index, int type,
> > > > > -			      u64 cpu_addr, u64 pci_addr, u64 size);
> > > > > -int dw_pcie_prog_ep_outbound_atu(struct dw_pcie *pci, u8 func_no, int index,
> > > > > -				 int type, u64 cpu_addr, u64 pci_addr, u64 size);
> > > > > +int dw_pcie_prog_outbound_atu(struct dw_pcie *pci,
> > > > > +			      const struct dw_pcie_ob_atu_cfg *atu);
> > > > >  int dw_pcie_prog_inbound_atu(struct dw_pcie *pci, int index, int type,
> > > > >  			     u64 cpu_addr, u64 pci_addr, u64 size);
> > > > >  int dw_pcie_prog_ep_inbound_atu(struct dw_pcie *pci, u8 func_no, int index,
> > > > > -- 
> > > > > 2.25.1
> > > > > 
> > > > 
> > > > -- 
> > > > மணிவண்ணன் சதாசிவம்
> > 
> > -- 
> > மணிவண்ணன் சதாசிவம்

-- 
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